F5 ASM Signature Could Not detect XSS Attack

Marking this as answered as the issue was raised as an SR with F5 Networks Support and addressed in a subsequent ASU release.

It is recommended to update the Attack Signatures on an ASM/Advanced WAF device when new releases come available for up to date protection and enhancements in detection methods.

From 13.1.0.4 ASM with updated Attack Signatures (Update: v13.1.0/ASM-SignatureFile_20190114_163855):

Detected Keyword    
text=*/1:eval.call(0,atob(YWxlcnQoZG9jdW1lbnQuZG9tYWluKTs=))})//
Attack Signature    
Signature ID
200001324

Signature Name
 eval() (Parameter)

Context Parameter (detected in Query String)
Parameter Level Global
Actual Parameter Name   text
Wildcard Parameter Name *
Parameter Value */1:eval.call(0,atob(YWxlcnQoZG9jdW1lbnQuZG9tYWluKTs=))})//