I'm using Debian 10.7 with MATE DE [will be updated later this week]The machine is a Lenovo T510 and is setup to login as either "richard" or "root".
If logged in as "richard" I can execute su {+ password} and receive a prompt indicating I'm "root".
However if I then enter "update-grub", the response is "bash: update-grub: command not found" as if I were the unprivileged user "richard". All is normal if I had initially been "root" or had become "root" via System -> Log Out richard . Bug?