The general conclusion I draw from this:
Editing a problem in a homework set which you have not assigned to yourself can lead to some bizarre behavior.
When the library browser creates a homework set it automatically assigns (actually it asks first) that set to the creator. If you mainly create sets through the library browser as many do you don't run in to this behavior.
This knowledge gives one a work around for this user interface bug. It's not clear what the fix should be -- it might become a bit complicated. One possibility is that you aren't allowed to edit a set which you haven't been assigned to. Attempting to edit the set automatically assigns you to the set.
This doesn't seem like the best approach in terms of "principle", but it might work in practice and would be easiest to code.
Handling questions which have not been assigned to sets and sets which have not been assigned to a user is currently done on an ad hoc basis in each portion of the code so inconsistencies arise.