When running and debugging a failure on a branch that implements mirrored tables in test/format it detects a torn transaction (pretty quickly) with the attached CONFIG.1 when verifying a backup.

It was only very recent that test/format got the ability to operate on many tables. What this branch of test/format is doing is selecting some subset of N tables to mirror each other on updates. When making those mirror updates to multiple tables, the test is doing so within the same transaction. So all mirror updates should be identical across all mirror tables. That is the failure we're detecting right now, two tables have different values for the same key.

To be clear, in this case, the backup is acting as an offline checkpoint/recovery verification mechanism. I have no reason to doubt the backup code itself either within WiredTiger or the test.

I will put details of the failure and reproducing steps in the comments since there is a lot of information.