Generally for me, in the 2nd (might be 3rd) section there's a brood mother that comes out that's on a steep hill. It doesn't happen all the time but if you throw turrets on the hill on a steep section I'd say about 1 in 3 seem to go into the map or just disappear.
I just noticed it happens way more often on that map, not so much on the others. It seems random so could be that the first is fine but your second turret just disappears.
By the looks of it I think it falls through the map itself in my experience, can't say what other have experienced. I would say though bar that map it's not a massive problem.
Hope that helps in one form or another Toby.
It does indeed - we were able to reproduce the behaviour of the turret falling through the steep hill in the mountain outpost.
We're also investigating a separate turret related issue where by turrets can dissapear if thrown too closely to your feet.