MTXW UBEs on V6R1

Is it normal to have UBEs go into MTXW momentarily before processing?

For the most part yes in my experience. I've typically seen this when the global table and DD TAM files are JITIing in. However, once you run a few jobs after a package deploy this status should be all but non existent. When we first installed 8.12 we saw this a lot but then soon realized we didn't need to journal our spec file folders in the IFS for our replication software and and the 1-2 second MTXW status went away and our very short running batch jobs saw some significant performance gains. Check you don't have anything in the spec directory journaled or replicating besides the spec.ini file.