I've actually noticed this for a couple of builds now but have been patiently waiting to see if it resolved itself. I'm currently using the latest 2.1/3590 version.
It seems that the user list of chat participants does not keep in sync with the actual participants in the chat room. Quite often a user will be displayed as present in the room when they clearly are not. Sometimes when they rejoin there are duplicate, but not exact names (one will be displayed normally while the other will have a "+" in front of the name).
I'm not sure when this started exactly, but it was about the same time I started noticing the status symbols in front of the user names (+, @, etc). Not sure if they are related.
My apologies if this is a dupe. I did some searching and wasn't able to find anything that resembled this particular issue.