I investigated this a bit more and effectively there’s issues in the way geometry collection initialize the physics state and collision that conflict with setting the rest collection at a later stage
Unfortunately there’s no easy code workaround for it ( It’s on my list to fix )
For the time being you will need to spawn geometry collection with a predefined RestCollection
A way to do that could be to set Blueprints that inherit from GeometryCollectionActor and set the rest collection of them
Then you can have a blueprint that basically spawn those GeometryCollectionActor blueprints on demand ( depending on your logic )