Why did stateful session beans (SFSB) change transactional behaviour in EAP 7

Issue

• Custom PhaseListener with UserTransaction fails with ConcurrentAccessTimeoutException error:

  WFLYEJB0034: EJB Invocation failed on component Resources for method public javax.persistence.EntityManager org.jboss.as.quickstarts.tasksJsf.Resources.getEm(): javax.ejb.ConcurrentAccessTimeoutException: WFLYEJB0228: EJB 3.1 FR 4.3.14.1 concurrent access timeout on Resources - could not obtain lock within 5000 MILLISECONDS
  at org.jboss.as.ejb3.component.stateful.StatefulSessionSynchronizationInterceptor.processInvocation(StatefulSessionSynchronizationInterceptor.java:94)
  at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:240)