Yes, it is in principle possible that Apple was doing something special to support bad PDFs produced by ABBY which they then removed.
If this is what happened, I would agree with you.
But, there is no evidence yet that they removed anything.
It’s quite possible (I’d say likely) that ABBY’s PDFs were corrupt all along.
Preview may have tolerated them not because of a special feature which was then removed, but because it happened not to depend on correctness before.
In this case, Apple wouldn’t have been in a position to even know about the problem.
You know who would? ABBY, who have had free access to Big Sur for months. You’d think that even cursory testing at their end would have shown this problem.
They have had months to fix it, months to work with Apple on the issue, any months to publicize it it really is Apple’s fault.