My user had spent several hours on a comment. They went to expand the issue (the double sided arrow icon in the top right) so that they could have more room to view the table they were creating.
JIRA warned them that doing so would cause them to leave the page and their comment would be lost. So they clicked cancel. It warned them again with the exact same message and they canceled again. But it proceeded to partially expand the page anyway. By partially, they mean it made it blank, but expanded. They went back to get to their comment. It was gone.
I was able to validate the same behavior. Looks like a bug in the software. You should submit it to Atlassian.
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