I am getting the subjected error code. This code is occuring only with one specific URL. It is working fine in one ASM Policy with 11.6.1 It is not working and generating support ID with error (illegal http status in response: 403) on other ASM Policy with 12.1.1.

response code 403 is not included in default allowed response code:

https://support.f5.com/csp/article/K7922

this is the normal behavior to block as a violation in version 11.6 and 12.1.

if it is not blocked in version 11.6, you may have unchecked "Illegal HTTP status in response" in blocking settings page.