We have a straightforward Time to First Response SLA set:

In most instances it seems to work. However, on random tickets, the Time to First Response clock doesn't stop when our agents leave a comment within 18 hours. We are on JSD Cloud, not server. I can't seem to find anything consistent across the tickets where it fails to stop, either.
EDIT: Could this be the issue?
One of our agents has multiple user accounts showing up in Atlassian. Although he's responding from his JSD agent account, his name appears in jira as what might be recognized as a customer account.

I looked under our JSD "People" and his gmail.com account isn't listed there, but I do see it under "Customers" without the ability to remove it.