Why is the PAM configuration not working properly? [closed]

Question

I attempted to edit the /etc/pam.d/su file to achieve the following:

• When the current user is userX and executes su userM, no password is required. I used the following configuration:

  auth [success=ignore default=1] pam_succeed_if.so user = userM  
  auth sufficient pam_succeed_if.so use_uid user = userX  
  

• When the current user is userY and executes su userN, no password is required. I used the following configuration:

  auth [success=ignore default=1] pam_succeed_if.so user = userN  
  auth sufficient pam_succeed_if.so use_uid user = userY  
  

The strange issue is that if I configure only userX --> userM or only userY --> userN, both work as expected.
However, when I configure both sections simultaneously, only the latter configuration (userY --> userN) works. When trying su with userX, I get the following error:

su: failed to user credentials: The return value should be ignored by PAM dispatch  

Did I make a mistake in the configuration somewhere?