From the first 12.0.1 and 12.1 right now on my MBP 2021 ARM . I always got the TM issue where it backed up and then it doesn't really complete the backup.
What I did to fix it permanently is to first do a backup to the disk. While the screen is awake. Until it's done. Don't skip the cleaning up process. Let it run. If you don't want to worry it having the screen off just use Amphetamine. When it's done it should be fine. You'll see the last backup date and confirm that it works. After that it should be able to backup every time even with screen off .
And I've been able to reproduce on how to make it problematic again.
If you,
1. Install a new MacOS Update
2. Use Onyx with the Maintenance tab which cleans things.
It'll happen again and to fix it you can do the usual again.
This has been consistent for me and I didn't even need to add any exclusion etc to the TM . Although I don't know if 12.2 fixes it. We'll see
What I did to fix it permanently is to first do a backup to the disk. While the screen is awake. Until it's done. Don't skip the cleaning up process. Let it run. If you don't want to worry it having the screen off just use Amphetamine. When it's done it should be fine. You'll see the last backup date and confirm that it works. After that it should be able to backup every time even with screen off .
And I've been able to reproduce on how to make it problematic again.
If you,
1. Install a new MacOS Update
2. Use Onyx with the Maintenance tab which cleans things.
It'll happen again and to fix it you can do the usual again.
This has been consistent for me and I didn't even need to add any exclusion etc to the TM . Although I don't know if 12.2 fixes it. We'll see