did you read the wiki on for example CLIENT_ACCEPTED (CLIENT_CONNECT doesn't exist)
https://devcentral.f5.com/wiki/iRules.CLIENT_ACCEPTED.ashx
because of how UDP works it might trigger on different moments, looking at it and if you have regular traffic i could imagine this event will never trigger because it never expires, so the event doesn't trigger.
not sure if you can test, but if you can perhaps the LB Datagram option can be enabled to see if you get more logging then.
also i don't quite get your last statement, does the virtual server break when an irule that only logs? could you provide the irule code here?