@Knight From my experience, the "last seen" in the user's profile seems to be a more reliable indicator than the "last seen" in the chat profile.

I can also confirm from my own experience that often when I click leave on some chatroom, I am then still shown as present in that chatroom.

So the fact that I am shown among the users currently in some chatroom does not always mean that I am there.

The fact that leave does not always work was also mentioned in some posts on Meta Stack Exchange: Leave a chat room, but not really and Not able to leave a chat room.

@MartinSleziak Thanks for replying.