After suspecting foul play, I decided to pause the ARKSView scene after noticing my CPU usage was too high after the moment where the scene is supposed to be dismissed.
Sure enough, pausing the scene stops the rendering for that scene and the other scenes load properly instead of behaving as if they were paused. (It chugs along until an ad loads and the scene comes back, then the app crashes)
(My guess is this is related to resources not being released by the ARScene coupled with some kind of limiting mechanism done by iOS)
Once I figure out how to dismiss it properly I'll post an update
So after even more experimentation, my conclusion is that I'm either doing something wrong or ARSKView and SKView are bitter enemies.
Even adding ARSKView to an empty SKView causes the ARSKView to experience a resource choke. (Still functions)
At this point I've abandoned all efforts to make them work together, and I'm basically replacing my iOS target with a fresh ARKit target, because the old version of the game is better off just being the Watch version anyway.
So, not a solution, but definitely a conclusion.
Hope this helps!