Tagging a user in a reply to a comment on an open task in an unmerged PR in the "Comments on prior versions" window makes the window disappear

This issue bears similarity to issue BCLOUD-12073, but is more significant because it is regarding work that needs to be done on an unmarked PR. I am waiting on feedback from a teammate on work I'm doing. He had wanted me to rework the code in a different way and there were 3 related tasks for doing it. I did the first one, which I assume made the 3 tasks linked to a "prior version" of the code. When I comment on the second task, I want my teammate to respond, but when I try to tag him, the comment window disappears. Here's a screenshot: