When using the EJB remoting transport that layers over the top of JCA it is possible for a race condition between the two reapers.
Normally you have server 1 and server 2. Each of these insert the TwoPhaseCoordinator in their TransactionReaper. If the TransactionReaper in server 2 fires first then it will rollback the TPC.
When server 1 Reaper EJB remoting transport fires it will try to rollback at server 2 also and it is this that should be safe to remove the transactionImple reference.
At the time of reporting the reference is not removed though and so the reference to the TransactionImple is not removed.