# IC degraded due to being manufactured looong ago (~35y+)
# Bad manufacturing batch
# Storage conditions
# Chip at some point pulled from other systems/boards? (which had previous owners)
# Fake chips
# ESD damage
1) Pulled from, or sitting on boards that were built much later (~2010?) than AY-3-8913 original production. There's no clear indication how makers of those boards obtained their ICs. So "bought from random eBay seller, potentially fake or relabeled chip" is entirely possible. Not excluded, anyway. Even if seller was legit, other options like poor storage, ESD damage etc still apply.
2) One vintage board with original IC. This of course pre-owned, IC not tested outside of this board, and... sample size 1.
Then this link:
https://www.bytedelight.com/?p=6327
Same thing: "AY-3-8913 chips we bought may have been duds or fakes".
If you'd find some gear that actually used these AY-3-8913s back in the day, in unmodified state, with a known history (use/storage/previous owners), pull the AY-3-8913s from that, test in isolation, and then also see the same problems, then you'd have a case.
But with the data as presented in article: could be anything, really.
The only thing I take away from all this: if you're building something that uses an AY-3-891x, stay away from the -8913. Because they're rare / hard to find, and "buy from eBay" has a good chance of getting you a bad chip. Dud, fake, re-labeled or otherwise.
Which is good to know but says little to nothing about genuine, original AY-3-8913.