I just updated our gitlab to 10.8.4-ee (f80a528). It’s entirely possible this may not be related to the update, but we have a few devs with a very large merge request (267+ changes, it’s a monster of a merge request). On one portion of the discussion, where there were previously comments on a few lines of code, the code has now disappeared from the discussion and any new replies briefly appear and then POOF, disappear. So new replies won’t show up on those.
I can go in and make any test comments I want on most any other portion of the discussion, but these handful of others won’t let anyone reply. You can, however, resolve the discussion, but comment replies disappear.