Ok, I found the bug. As usual in these cases, "stupid bug"
Consider it being fixed for the next release. Actually, the bug triggered a couple of other things that might been strange, like deleting a rig, but still appearing in the list (in case it was assigned twice).
To answer the question regarding multiple assignments: You could select slot #1, press assign, select slot #3 press assign again and the rig is assigned to program change 1 and 3. erasing the rig will leave 1 and 3 unassigned, changing a rig will "update" 1 and 3 (since it is just a link to the current rig). On the Browse page the MIDI number only shows the first assignment in this case ("1").
Timo