Time Machine always makes a full backup
Backups are managed via Apple File System Events (not Spotlight)
https://en.wikipedia.org/wiki/FSEvents

It seems most likely that something is touching your files in a way that convinces TimeMachine a new copy is needed. There could also be something damaging the FSEvent cache which would cause TM to create a new backup.

You might want to check the system console log and see if there are any messages there that hint at the underlying problem.