I see the tm file stores being created in whatever the working directory of the server happens to be rather than under the jboss.server.data.dir. It appears that in the absence of a configuration like the following:

<subsystem xmlns="urn:jboss:domain:transactions:1.0">
...
<object-store relative-to="jboss.server.data.dir" path="."/>
</subsystem>

that the TransactionSubsystemAdd should be defaulting to a location under the jboss.server.data.dir, but that does not appear to be working. Either the default needs to be added to the configurations, or corrected in the TransactionSubsystemAdd code.