this is a strange finding, looks pretty special.
Can you rule out this is caused an App that is (probably) not compatible anymore?
In case you have the chance to announce a small timeframe for a downtime-like situation, I'd suggest to disable all Apps by putting Jira to safe mode. Then you could check if the dashed line still occurs.
If so, it is not caused by an App.
If not, you could re-enable Apps, one by one and check when the line occurs again.
If it is not caused by an App digging deeper into it - probably using developer tools/view of the browser could be the next steps - but please report first back what you found out.
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