@Epic, no it doesn't and shouldn't close the chan tab if you have "Keep Channels Open" checked.
Yes, it should... the "Keep Channels Open" feature was added long ago to keep channels open if you were kicked from a channel or disconnected from a server, not if you chose to /part a channel.
However, it looks like ZNC is using this method to "detach" a client, ie. a non-user-initiated PART.
The question is whether changing the "Keep Channels Open" feature to keep channel windows open even if you /part a channel would annoy users who have this feature enabled. It certainly would annoy me :-]