How to reproduce:

1. Make sure course completion is enabled on the site.
2. Create 3 courses with following completion tracking settings:
   1. Course 1 - completion tracking enabled
   2. Course 2 - completion tracking enabled
   3. Course 3 - completion tracking disabled, note ID of the course.
3. In Course 1, navigate to "Edit" -> "Course completion", expand "Condition: Completion of other courses".
4. Press on "Courses available" search dialog, observe that you can only see "Course 2"
5. Select "Course 2"
6. Open dev tool, find "select" element with name="criteria_course[]", replace value for "Course 2" option with ID from "Course 3", save the form.
7. In Course 1, navigate to "Edit" -> "Course completion", expand "Condition: Completion of other courses", observe that currently selected course is "Course 3"

This has been discovered while writing a testing instruction for MDL-68315 (in order to replicate issue you can follow that test instruction). Basically the issue is partially results from MDL-68315 (Course form element does not respect 'onlywithcompletion' attribute), but in addition to that verification is missing on submission which ideally should check that selected courses have got completion enabled.