Propose:

Add a README file in /etc/sudoers.d to provide information in regards to excluded filename, i.e.

The @includedir directive can be used to create  a  sudoers.d  directory
that the system package manager can drop sudoers file rules into as part
of package installation.  For example, given:

   @includedir /etc/sudoers.d

sudo  will  suspend processing of the current file and read each file in
/etc/sudoers.d, skipping file names that end in ‘~’  or  contain  a  ‘.’
character  to avoid causing problems with package manager or editor tem‐
porary/backup files.

Files   are   parsed   in    sorted    lexical    order.     That    is,
/etc/sudoers.d/01_first  will be parsed before /etc/sudoers.d/10_second.
Be  aware  that  because  the   sorting   is   lexical,   not   numeric,
/etc/sudoers.d/1_whoops  would be loaded after /etc/sudoers.d/10_second.
Using a consistent number of leading zeroes in the  file  names  can  be
used  to avoid such problems.  After parsing the files in the directory,
control returns to the file that contained the @includedir directive.

Users from AD via trust are often with a dot in username. It is a common mistake to put a drop-in file with username and the file is being ignored due to "." included in filename.