Hello there,
while fighting Mono boss in Reign of dragons with my bladedancer, i was able to port outside of arena boundaries (invisible wall) and was unable to move.
So far I found 2 spots, from 1st spot you can actually port outside (must be hole inside invisible wall), but from second one, you can just port from Mono.
Here is video showcase: Last epoch - 0.9.0 - Reign of dragons arena bug - YouTube