That is odd, and definitely not the default behavior.
It would lead me to suspect something up with either permissions to that role that have been changed, or perhaps something upstream where some of these users have the permissions from some other role. If you create a new course from scratch, set force Separate group modes on, enroll a normal test user as Non-editing teacher and the problem persists as this test user, then I think there's an issue with the role definition in general. Something is different in those two courses from the other ones.
It would lead me to suspect something up with either permissions to that role that have been changed, or perhaps something upstream where some of these users have the permissions from some other role. If you create a new course from scratch, set force Separate group modes on, enroll a normal test user as Non-editing teacher and the problem persists as this test user, then I think there's an issue with the role definition in general. Something is different in those two courses from the other ones.