I got exactly the same. Multiple units in persistence.xml and they vary every deployment. I havent solved it yet.
I would like to make sure we get this right for AS7. I started a test here https://github.com/scottmarlow/jboss-as/commit/d704226cca85d8e870f5ac91254bc816c0dd2475 that is not yet done (hopefully with be soon).
Looks like we are good in AS7.
The AS7 unit test for the above issue, is passing. Source is at https://github.com/scottmarlow/jboss-as/commit/c3ebdf0651aa91e92a0a35a018a3502943a6e1af
I also added some trace logging to help see what is going on during the search for the persistence unit: