I was wondering if Splunk had any 7.x Universal Forwarders that had known problems or bugs such as not sending logs even though service is running, or other similiar issues that could cause the process and or service to crash?
I haven't heard this kind of bug for UF. In this kind of situations there is usually some configuration issues on UF or indexer side.
Can you give more information about your environment and also inputs.conf, outputs.conf etc. so we can try to help you?