AP: 8.1C(100810). Could not find release date.
BR: 8.1C(110512).
above info taken from help / about.

I have to go into AP Company Profile, Batch/Vendor tab, that "Alphanumeric manual check numbers" and check the box last month due to this issue.
and this month, I have the same issue and have to check the box again.

I am pretty sure (99% sure?) that we did not change the setting in both cases.