I have multiple villagers that will just randomly change professions, even after i have traded with them to set the profession desired.  I make multiple trades with them, and then some time later will check back with them, and they have switched professions, and are now locked into a profession that was not the one I traded with.  I have had this happen on multiple villagers.  I can open the trade with them, and see their experience has gotten to where it was before the change, but they seem to be locked in after the final change.