...and a DR plan isn't worth the paper it's written on unless you test it regularly (people leave, media gets corrupted... just like the military; do drills!)
... and a DR plan isn't worth anything unless you test the "people leave" part without any knowledge in their heads. your internal documentation and credential storage needs to be solid in case four people flying together all die abruptly in the same plane crash. the traditional "bus problem".
Compare an older snapshot to current for unexpected changes? Though that is difficult (probably not practically possible in the general case) to automate because telling the difference between rare but expected changes and unexpected ones due to corruption could require quite some intelligence/intuition.
No backup+test stratergey can catch everything. You just have to be sure to catch everything that you practically can (and need to factor your time & effort and the importance of the data, into the judgement about that is practical to do and what is justifiable or unavoidable risk).
In the general case, but in this case isn't unlikely at all: you have a ZFS pool that suffers from this bug, you back it up, the missing files doesn't show up neither in the original or in the backup.
... and backups are not backups until you can reliably restore from them.