I also have the collision type set to Implicit-Implicit. I see you said you have set the Level0:Destructible and Level1:DestructibleOff. I wasn’t able to make it work like that, I had to use the following configuration:
Level0:Destructible
Level1:Destructible
Level2:DestructibleOff
And create the geometry collection with these 3 levels, making sure that there weren’t any unclustered items isolated in level 1 (basically: all leaf fragments must belong to level2).
For my specific setup I have programmed a custom actor that wraps a GeometryCollectionComponent and handles the received hits. By default my geometry collection doesn’t have the SetEnableDamageFromCollision enabled and I enable it only after receiving a hit. So with this behaviour I make sure on the first hit the level 0 is broken into several level 1 pieces, and then these pieces still have collisions and break down. If they fall and collide with something else or receive another hit, they break into smaller level2 pieces, and these are the ones that have no collision with the pawn.
As an extra step to improve the feeling and add some reactivity with the player, I’ve also added a volume in the character that does collide with these pieces so they move when the character walks through them.
This is what works for my specific setup though, maybe you need to set it up differently according to your needs.