This appears to occur more often in the Labyrinth even if it did happen before, both in delves and dungeons. It hapens often when you change lvel like walking up a stair and suddenly you pet dont have a clear path to the mob youre fighting and you do close to no dps at all.
There is a very simple solution to this problem. If you counter this, lets say three times in a row, the pets will teleport to you.
Problem solved.