Well, I came on to log an issue I am seeing with PublishQueuePosixRK - namely a single queued event is sent 2-4 times when called from setup(). Maybe related to what you have seen with PublishQueueExtRK. Definitely something not right since extended event data implemented.
I added a new example 3 to test running from setup and testing with 1 or 3 events worked for me.
You definitely can't publish before calling PublishQueuePosix::instance().setup(), but other than that I think it should work.
Do you have a trace log of it happening? Does it always happen?
I have moved creation of the event to a state in the loop() and this solves the problem. I will revert it and post a log of the startup where the event is created once and then sent/de queued a couple of times.