Are the users you mention getting other emails from JIRA? And can they see the issues they are being mentioned on? (If a project permission or issue security level blocks them from seeing an issue, JIRA won't leak issue information to them by emailing them)
If they can see it, and are getting other emails, then you're right in wanting to go to the logs to find out what it's doing. That requires Atlassian Cloud Support to do it though, so you'll need to raise it at https://support.atlassian.com
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