Preimage doesn't return same fields in different situations

Hello, I'm having a problem where the Preimages on a Pre-Operation plugin and a Post-Operation plugin don't return the same amount of fields. on the same Entity.

The Pre-Image is set to return All Attributes, and in one instance returns 72 key-value pairs, and in another, it returns 67 key-value pairs.

Anyone got any idea why this might happen?

in InstanceA we are trying to Assign a Case entity to another SystemUser, and in the other we are just saving it without doing much else.

in InstanceA 72 key-value pairs are returned in the code, whilst in the normal save, only 67 return.

Thank you!