I was losing my mind over this, so here it is:
Set a “default” Rest collection in your Geometry Collection component to get break events (and make sure “notify breaks” is checked in your geometry collection component).
Context:
I had a Blueprint with a Geometry Collection component with no Rest collection assigned. I added a Geometry Collection variable I can use to set the Geometry Collection component in the construction script using the Set Rest Collection node. I made sure “notify breaks” was on. Still no break events. Adding a “default” Rest collection (even if I end up reassigning it in the construction script) fixed it. I created a simple cube geometry collection which I use as the default value. Hope this can help someone!