The reaction seems like it's simply left the doc object constructed across the same instantiation of the NotesDatabase.
If the doc object remains loaded, it'll remain what it was last cached.
Given c#, I couldn't tell you whether a recycle() alone would solve the problem, but it's possible it would.
Y'probably need to experiment with calling recycle() on the doc and the col.
Failing that you may need to delete the doc object itself. But I haven't had to do that for a long, long time.