Note: I discovered this in our own moodle 3.11, and got the same results in "Mount Orange" at https://school.moodledemo.net/ which I guess is Moodle 4.1.2 (moodle version is sometimes hard to find)

1. Use (or create) a three step custom scale, for instance Bad-Medium-Good (as in Moodle test site Mount Orange)
2. Overall grade category Aggregation: Simple weighted mean of grades
3. Add an assignment, using the custom scale, Marking guide and No specific grade category
4. Create the Marking guide with one single critera with max grade 3
5. Click to save the Marking Guide ready to use
6. Control that you get the message "WARNING: Your marking guide has a maximum grade of 3 points but the maximum grade set in your activity is 2 The maximum score set in your marking guide will be scaled to the maximum grade in the module.
   Intermediate scores will be converted respectively and rounded to the nearest available grade.
7. If you don't get the message immediately, go back to Advanced grading to see the message.
8. When grading and setting the grade 1/3, verify that the grade Medium (2) is showing, when Bad (1) should be showing.

Additional info:
I have also set up an Assignment graded with Rubric, with three criterias and the grade 0 in all the criteria levels, except for the last critera, with 3 levels and the points 1, 2 and 3.
There I got no warning, and the grade was displayed correctly when using it.

There is a discussion here: https://moodle.org/mod/forum/discuss.php?d=445219
This all started from a discussion on "Holistic grading Rubric", https://moodle.org/mod/forum/discuss.php?d=444868