I observed that a user previously was among the owners of a chat room, but after some time, s/he disappeared from the owners list.
How can that happen?
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Any room owner can add or remove owners to/from the room, including themselves.
So my first guess is that they didn't want to be owner anymore; I guess you could ask them.
There's no automatic removal of room owners, so someone must have clicked a button.
Edit: Changing an owner (i.e. adding/removing a user from that list) is important enough for everyone in the room to know, so from now on, an automatic message will be posted into the room when that happens.
As came up in comments with balpha on that answer, this is about you being removed from the owners for the Lounge. I removed you because I was afraid your pranks (see discussion a little above and continuing past https://chat.stackoverflow.com/transcript/message/753110#753110) might run to shenanigans with the chat room mechanics. Of course, not being listed as an "owner" doesn't affect use of the room for chat, nor did I intend to do so.