In timeseries_retryable_write_downgrade.js, after downgrading the replica set, the original primary is required to become primary again so that we can perform an insert using the existing session. However, the current code that attempts to do this does not properly provide this guarantee. timeseries_retryable_write_downgrade_oplog_rollover, which is based off of this test, has updated logic for this which should work correctly.