You guys with the recovery updater problems: did you convert from HFS to APFS with Disk Utility and thus don't have a recovery volume inside the APFS container? I have a normal APFS layout (main volume, Preboot, Recovery, VM) on my iMac7,1 since I installed HS from scratch, and I didn't have this recovery updater issue.
If you don't already have the recovery volume inside the APFS container (or any of the other volumes HS expects on APFS, do a 'diskutil list' in Terminal to be sure) you could try the following in Terminal to add those volumes and see if it works with the updater:
Where X is the disk number of the APFS
container not your main HS volume:
Code:
diskutil apfs addVolume diskX APFS Preboot -role B
diskutil apfs addVolume diskX APFS Recovery -role R
diskutil apfs addVolume diskX APFS VM -role V