This sounds like it has something to do with the switching users code. I don't know how it is done, but I know it is in spamd somewhere, and uses Storable to store the user configs. There is special stuff (I believe) somewhere that is supposed to get invoked when switching users in a database case that is a little different than the non-database case. I may be wrong on that though.
I'd think your best bet for an answer would be Theo or Michael, and I'm surprised one of them didn't respond. Loren