How to fracture your object according to pre-existing polygroups or materials?

There doesn’t seem to be a way to do this yet -without- first fracturing the geometry. However, once fractured, individual meshes will have convex hulls generated for them correctly, with each piece separate.

The only workaround I currently know of would be something along these lines: use planar or similar to create a tiny cut on a corner, and then use the merge small pieces feature to merge it back to its parent. This should let you do what you’re after.

It’s probably something that’s been overlooked by Epic for now and might be worth a feature request, especially considering it shouldn’t involve much based on how it already works.