I fixed some issues around this exact problem in:
https://github.com/discourse/discourse/commit/c5610a179a3470fd38d5c82b895481a821fdfa74
Specifically the consistent repro of the issue was to have 2 conversations going and to flick between them.
You are … almost… at that commit, the commit should land middle of this week.
Just confirming though, when you reload the page after all stuff is done are you seeing stuff show up correctly?