I haven't tried it with several people on one server yet, but that's just what I noticed. When I go offline, I stay in the members list as long as I don't switch channels / servers or to favorites and dms, but as soon as I change my status to online, I appear again. When I'm in dms for example and change to offline and go to my server, I also don't appear in the list.
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Comment by @Werhas