Any chance that your issue is a variant of the Gnome desktop drag’n’drop problem discovered in with Fedora flavored Linux?
The offset between the dual pointers while dragging and difficulty determining the real hotspot often gave the illusion the drag’n’drop was non-functional. With Gnome, there were 2 pointers. With your desktop, maybe the offset is bigger or the 2nd pointer is invisible?
Gramps has fairly tiny drop target zones for each type of object being dragged. Dropping outside a valid target does nothing.