That's kind of a basic (i.e. dumb) reply. You must be a robot.
If you actually read what I wrote about the problem, you'd understand that I already know how to edit and save a contact my problem is with Contacts app software: it does not save my change.
I figured out one solution: delete the Contact that has the problem and enter it again as a new Contact. Then the Contact Custom Label is saved.
would help if Apple Engineers fix their software instead of guiding to solutions that have nothing to do with the reported issue.
no further help needed for this one.