Areas do not stop collision so there is always a possibility the player “falls through” it, either meaning being full enclosed or actually falling through it by one frame step, typically you will try to set reasonable speed limits and if you need some objects to move faster use larger-than-you’d-think collision shapes to cover any physics step gaps.
How are you testing the the box ignores the trampoline? It’s worth nothing the area is a child of your box so it will always transform with the other, if they aren’t overlapping at the start then they will never overlap, and if they are overlapping at the start then it will only apply such an impulse once at the start of the game.
If you are placing two separate box/trampoline instances then they should collide and bounce on each other.
get_slide_collision_count and other get_slide_ functions do not exist on RigidBodies only CharacterBody.