Not seeing all http 500 responses

Have you checked your ASM logs to see if it is blocking some of those 500 error responses? 503 responses are typically allowed to pass through the BIG-IP system but other 500 responses may be blocked depending on how you have ASM configured. Although the following article applies up through ASM v12.1, I'm checking to see if it applies to v13.0 also. (I have no reason to believe it does not apply.) K7922: Overview of BIG-IP ASM HTTP response code filtering