This is applicable for ESXi 6.7 - VMware Knowledge Base​. Currently, the only workaround is to manually restart the SNMP service as and when it stops. As per the known issues in the recently released ESXi 6.7 U1, looks like this is still an ongoing issue and might be fixed in the upcoming releases - VMware ESXi 6.7 Update 1 Release Notes​

Cheers,

Supreet