Hi Jack,
I have had issues with syslog before on a server. It required quite a bit of troubleshooting, with no clear solution to fixing the problem. The best thing is to create a ticket with Azure Support to figure this out:
https://azure.microsoft.com/en-ca/support/create-ticket
You could always use the troubleshooter steps yourself:
Troubleshoot your CEF or Syslog data connector
https://learn.microsoft.com/en-us/azure/sentinel/troubleshooting-cef-syslog?tabs=cef
If this is helpful please accept answer.