I was battling this last week with a chat group of 16 members. My workaround was to hide the group by getting everyone to leave, aside from myself. After 20 minutes or so, the group chat icon changed from 16 to 1, although all the members were still showing up in the list.
But earlier today someone found a mention/notification from earlier in the group’s conversation, inadvertently replied to it without realizing, and that immediately resurrected the group. The icon shows 16 again.
This afternoon, I built a new Discourse instance to verify it wasn’t database or plugin related, and the problem easily reproduced.