I've made this cake twice recently and both times the layer of apple has sunk in the middle so each slice comes out with a diagonal layer of apple from outside to in. The first time I thought the bottom layer was too shallow and I thought I undercooked it as the whole cake sunk in the middle a bit. The second time I made sure there was much more cake mix in the bottom layer and probably overcooked it, but the exact same thing happened, although the cake as a whole didn't sink, the apple layer still somehow migrated to the bottom of the tin during cooking.
The only I could think is that maybe I am creaming the butter and sugar too much, making the cake mix too light so it's not supporting the apple? Maybe it needs to be a denser cake?