I understand that the ideal thing here is to upgrade to 5.12 across the board, but there are many machines involved and that's not possible at the moment.

You should check if it was fixed by 5.8.9. Upgrading to 5.8.9 would be quite simple as it wouldn't require modules to be reinstalled.

Am I hitting something known?

You've shown that it was fixed, so it would seem it was known to someone.

I still wonder why.

Probably because the programmer overlooked something.