I don’t think the list is supposed to show which people are actively watching the chat, but rather people who are currently online and are “subscribed” to the chat, in the sense that they receive notifications if new chat is posted.
For the public chat channels that are not connected to a group, this lists any person who has the chat interface opened and for whom the chat shows up in the side menu. For example, with this test account I have opened the English and Off Topic public chats, thus my account shows up in the list whenever I have chat opened, and disappears as soon as I start doing something else on OGS (although the name stays in the list if the chat interface is open in another tab or in the background, so it isn’t related to keyboard focus or something like that):
As you can see, the chat name colours blue and has a little number next to it when there are new messages:
For group chat channels the lists works slightly different, since people usually receive notifications (in the top right) whenever new chat is added, assuming they have not set the group channel on ignore:
As a result, users are counted as “present in chat” whenever they are online on OGS, even if they don’t have the chat window open or are not on the group page. Basically, the list shows all users that are online on OGS and are members of the group. The only way to leave the channel, is to either go offline or to leave the group.
Perhaps this is not the most intuitive way to organise the list of users in chat.
I could imagine the situation could be improved with some icons next to usernames with three different colours to make a better distinction between users that are (1) online but not actively watching any chat channel (2) having a chat channel open in the background, or being focused on a different channel and (3) actually being present either on the chat channel or on the group page.
In any case, I don’t believe there is any bug going on here, and that the list functions as it was intended. I suggest you just simply ignore people who show up in the list while not reading the chat. It’s hardly an inconvenience after all. Perhaps you could check attendance it the manual way, and ask everyone to say hi before you start the meeting.