When a user performs an su command, /bin gets added to the users PATH before /usr/bin.

Issue

• When switching to another user with su, the PATH includes /bin by default:

  # echo $PATH
  /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin
  # su --login user
  $ echo $PATH
  /usr/local/bin:/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/home/user/.local/bin:/home/user/bin
  

• From the man page, we can read:

     -, -l, --login
            Start the shell as a login shell with an environment similar to a real login:
  
               o      clears all the environment variables except TERM
               o      initializes the environment variables HOME, SHELL, USER, LOGNAME, and PATH
               o      changes to the target user's home directory
               o      sets argv[0] of the shell to '-' in order to make the shell a login shell