It depends on where your character is.
Throughout my playthrough. I’ve noticed that Pokémon will evolve/hatch on the alternative screen in the following situations:
- On a slope
- On water
- in buildings
- crowded areas
Otherwise, the game will try to show it in the overworld.
Disclaimer: the game is still new, and this is my own experience. This answer may be missing certain situations. I was not able to find a source to verify these are the only situations.
I hope this helps!