Yeah I get that too, must be a bug I somehow found in c2. Have lost everything I just did then, nice. :/
Anyway the concept is:
1, Enemy animation 1 - make collider smallest line possible at bottom of sprite
2, Enemy animation 2 - make collider full size as normal
3, Pin an invisible larger box around player
4, When enemy overlaps that box, set enemy animation to 2
5, When enemy not overlapping box, set animation to 1
Basically the flat collider lets them pass each other, then they get bigger colliders when near a player.