I just did Sarven Savior (completed it), and it appears that some of the ally NPC’s have rather glitchy pathing. I followed the normal design of this level (send your allies back, build a fence to keep the enemies out), and one of my NPCs is getting stuck along the bottom mountain edge, just outside of the X area. To get around that, I had to move past the stuck NPC, then build 2 fences just behind them , in order to keep the enemies out. It seems to be Nikita who is having bad pathing.
Nick, you may want to double check that pathing