If pagure is configured to show comments via the SSE, but the SSE is no longer responding or you have too many connections to it (which is restricted/limited by your browser), you can send the comment but it doesn't show on the page.
We had a fix for this in 4.x where we should show the comment via JS instead, I believe we should bring this back.
Metadata Update from @pingou: - Issue set to the milestone: 5.1 - Issue tagged with: bug
Metadata Update from @pingou: - Issue assigned to pingou
Commit 485acb73 fixes this issue