hard drive used for time machine backup is not showing content when I downgrade from BigSur to Catalina

[newbtomacs]

I was running BigSur on a mid 2015 iMac, but realized that I needed to downgrade to Catalina to run a specific program. I bought a WD HDD, used time machine to back up my computer, and then successfully erased my iMac and installed Catalina.
However, now I can't see the contents of my WD hard drive with the backup (it showed up before I downgraded). The external "Element" is formatted as an APFS Volume, and I can see that there's 264 GB sitting on it. But I can't see those files or get Time Machine to recognize it now.
Any suggestions on how to make this work?

 

[newbtomacs]

If it's helpful, I ran first aid in disk utility and came up with this:

Spoiler: First Aid

Running First Aid on “Elements” (disk3s2)

Repairing file system.
Volume was successfully unmounted.
Performing fsck_apfs -y -x /dev/rdisk3s2
Checking the container superblock.
warning: container has been mounted by APFS version 1677.141.3.7.2, which is newer than 1412.141.1
warning: disabling overallocation repairs by default; use -o to override
Checking the space manager.
Checking the space manager free queue trees.
Checking the object map.
Checking volume.
Checking the APFS volume superblock.
The volume Elements was formatted by diskmanagement (1677.141.3.7.2) and last modified by apfs_kext (1412.141.1).
Checking the object map.
Checking the snapshot metadata tree.
Checking the snapshot metadata.
Checking snapshot 1 of 2 (com.apple.TimeMachine.2023-01-18-200138.backup)
warning: xattr_val: object (oid 0x2): invalid flags (0x6), given name (com.apple.fs.cow-exempt-file-count)
warning: inode_val: object (oid 0x15): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x16): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x17): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x18): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x19): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x1b): invalid internal_flags (0x428000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x1c): invalid internal_flags (0x428000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x1d): invalid internal_flags (0x428000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x26): invalid internal_flags (0x428000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x27): invalid internal_flags (0x428000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x28): invalid internal_flags (0x428000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x29): invalid internal_flags (0x428000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x2a): invalid internal_flags (0x428000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x2b): invalid internal_flags (0x428000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x2c): invalid internal_flags (0x428000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x2d): invalid internal_flags (0x428000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x2e): invalid internal_flags (0x428000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x2f): invalid internal_flags (0x428000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x30): invalid internal_flags (0x428000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x32): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x33): invalid internal_flags (0x428000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x34): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x35): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x36): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x37): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x38): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x39): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x3a): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x3b): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x3c): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x3d): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x3e): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x3f): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x40): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x41): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x42): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x43): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x44): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x45): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x46): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x47): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x48): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x49): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x54): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x58): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x5a): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x5b): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x5c): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x5d): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
warning: inode_val: object (oid 0x5e): invalid internal_flags (0x408000 / valid-flags are: 0x3fffdf)
Too many warnings of this type generated; suppressing subsequent ones.
Checking snapshot 2 of 2 (com.apple.TimeMachine.2023-01-18-212928.backup)
warning: xattr_val: object (oid 0x2): invalid flags (0x6), given name (com.apple.fs.cow-exempt-file-count)
Checking the extent ref tree.
Checking the fsroot tree.
warning: xattr_val: object (oid 0x2): invalid flags (0x6), given name (com.apple.fs.cow-exempt-file-count)
Verifying allocated space.
The volume /dev/rdisk3s2 appears to be OK.
File system check exit code is 0.
Restoring the original state found as mounted.

Operation successful.

 

[Chancha]

Something has changed in Big Sur as to how Time Machine backups are created / formatted. In short I don't think it is possible for Catalina or older to restore from a backup made in Big Sur onwards. At least I have not seen anyone doing that successfully.

Normally a TM drive even if you can't restore from it you can still read and copy the data via Finder. But since your case seems even that is impossible, the drive is formatted in a way that Catalina has no ability to access. This leave you the only solution... install Big Sur back on the Mac, restore from that TM backup, then at that point plan ahead what to do next knowing that limit exists. If you absolutely want to move back to Catalina you may better backup the data manually then copy back to a Catalina install old school way.