Having said that, PDF is vastly more complex than it was originally. It can contain forms, JavaScript, audio, video, accessibility metadata, etc.
It’s an extremely complex format.
Without further information this is more likely to be an Apple bug because they are doing the harder job.
Plus some further information:
ABBY has had months to support Big Sur but still haven’t.
That makes it seem like they either don’t care about supporting MacOS users, or they have a very low engineering budget.
Either way, that makes it seem more likely to me that they have a bug, than Apple.
But, regardless of the balance of probabilities, the point is that the original piece blames Apple entirely.
And yet nobody here really seems to dispute the idea that ABBY, a professional PDF generator could be producing a buggy file.