Catch 503 response code from F5

We are getting a lot of 503 (HTTP Responses) in our logging system(Kibana) for a specific app/vip.

I've added an Irule to the VIP to log the 503 responses into ltm log

However the amount of 503s its catching from the pool members is less then what the syslog server is seeing!

any ideas on how to catch 503 responses that F5 might generate on its own?

To be clear, the F5 seems healthy, cpu and memory levels are in the green

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