When update finishes, MDL_context::release_transactional_locks() first
releases MDL_STATEMENT locks (MDL_BACKUP_DML and MDL_BACKUP_TRANS_DML
in this particular cases), which allows FTWRL connection to proceed.
Then MDL_TRANSACTION locks get released (MDL_SHARED_WRITE in this case).
metadata_lock_info query may sporadically hit this gap and observe
these otherwise unexpected locks.