Hi @Cowseye
Please clear the configuration cache and perform an IISREST on the server to get all the SharePoint servers up to date on the latest farm information, check if the issue still exists.
If this problem persists, please check the precautions mentioned in the following article one by one.
https://www.sharepointdiary.com/2012/02/sharepoint-alerts-not-working-troubleshooting-checklist.html
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