I was pointed to the issue on the StackOverflow (https://stackoverflow.com/questions/47953827/defer-transactionalobservernotifier-fails-with-arjuna016082-synchronizations-ar) and I started to investigate it from perspective of transaction manager. I started the forum thread here: https://developer.jboss.org/message/985431.
Btw. my reproducer for WFLY testsuite can be checked here https://github.com/ochaloup/wildfly/commit/2ae984aacfb9b9171bf2e9c9dfb6c1ee4fdc1388