When I trade with villagers or hurt them to obtain non zero popularity and then disconnect from the server, my popularity in that village resets to zero. The same happens in single player if I move far away from the village. Hard to believe this to be an intended game mechanic because it makes it way too easy to get rid of bad popularity.

In a server the entire village gets deleted from villages.dat when there are no players present.

Affects 12w32a where the popularity mechanic was added and 1.9 pre-2. Presumably everything in between too but I didn't test all of them.