Running the new version of test format that supports mirrored tables will fail if restarted on an existing database with a CONFIG that specifies table.runs.mirror=0 for all tables. I.e., a configuration that has mirroring disabled.
What happens is that config_mirrors() will assign a mirroring relationship. But since the tables already exist and weren't mirrored to begin with this leads to (many) mismatch errors when format tries to verify the mirrors.
We encounter this problem when running format abort testing. Running format.sh -a causes format to abort (via the format.abort=1 setting). The script then restarts format to make sure WiredTiger can recover and continue. If this happens in a CONFIG without mirroring we hit this issue.
We haven't been seeing this regularly because abort testing with format was disabled in Evergreen due to
WT-9767. Note that the failure on that ticket looks like a different issue since its CONFIG has a mirroring relationship.