While fighting Jungle Lurkers in the Hohina Ravine on Tehiwai Islands I encountered a soft lock. My best guess is that Mage Slayer is successfully interrupting a spellcast but this is causing turn based combat to break. Unfortunately the combat log does not offer an indication of a spell being interrupted here, though it would if I was using the pet boar and still it would soft lock so long as there are spell disruption stacks. I have not encountered this problem before this point due to either luck or bursting down an enemy before or after their spells had been cast. This is on Veteran difficulty (solo, Eothas and Berath challenge) and can happen even with the pwgra if they attempt to cast something while there are spell disruption stacks.
A screenshot