Hello,
this is an issue that I noticed in any Jira version from 7.13 to 8.5 now.
You go into a scrum board. Then you click on Backlog and choose an issue of your choice. In a panel on the right, issue details are now shown.
Click on comment and add a new comment. After adding the comment, the comment section in this detail panel is fully empty. My expectations however are, that the just added comment would be appended to the list of already existing comments.
Can somebody reproduce or has a solution?