Debain 10 root Environment Variables не работают правильно

На самом деле работают.

По умолчанию все настройки переменных окружения лежат тут /etc/profile

Но, в [официальной документации] сказано:

Using su

 su command is used to become another user during a login session. It is commonly used to get root permissions temporarily from normal session. In Stretch and earlier releases, the su command resets your PATH enviro nment value to one defined in  /etc/login.defs by ENV_PATH and ENV_SUPATH variables. In Buster, su does not change your PATH variable.

In Buster, su does not change your PATH variable.

Решение: su - root instead of su root

[Источник]