It sounds like corrupted index or corrupted table.

That's what I initially thought too until I reproduced in multiple other data sets. I double-checked by running Abri data recover tools on the databases and no issues were found.

This bug only happens intermittently. One record may commit fine, but the next one fails.

No pending transactions... no anything. Just this sinking feeling that it's yet another bug related to tables being opened multiple times with the AGAIN ALIAS option.