They’re not stranded because the part of the capsule that isn’t working has multiple redundancy and is intended to burn up on reentry anyway.
Starliner is perfectly capable of leaving the ISS whenever they want, but they would be unable to continue collecting data on the thruster shutoff (again, because it would burn up in the atmosphere).
Glaze has always been fundamentally flawed and a short term bandage. There’s no way you can make something appear correctly to a human and incorrectly to a computer over the long term - the researchers will simply retrain on the new data.