Resolved the problem. The layout above works fine, the problem was with my Model. Items in the collection had unstable identifiers.

To test my hypothesis about the environment being the cause, I replaced my two custom UTIs with system UTIs (public.item and public.folder). The missing provider issues went away, my code was working but the system was not making the custom UTIs available to the DropDelegate. However, the issue I raised with the filter not working in onDrop(of:delegate:) still seems broken.