I think the problem is that onEvent() starts a new transaction and the UserTransaction instance is somehow reused. I'm not sure whether it's allowed or not (from JTA POV) but if it is it makes sense to clear its state (e.g. clear all javax.tranasaction.Synchronization registrations).

In any case, this is very likely not a Weld bug. Therefore, I'm going to reject this issue. Feel free to add additional comments and attach your simplified version of code.