TimeMachine backups failing on nonexistent file in excluded directory

I have a 2TB external disk for TimeMachine with 800+GB free.

Time machine fails saying some file can't be backed up. I go to find this file, and it's not there. I've excluded the entire folder containing the file and even the folder containing that folder. No improvement.

Because of this "Time Machine couldn't complete backup to "disk"" error on "/Library/.../excluded folder/ another folder/file", nothing gets backed up.

Why can an error on an excluded path even happen? How do I fix this?