relative altitude updates are still the same, I agree with you. However, absolute altitude updates are definitely not. Also, while I appreciated your suggestion, which makes total sense, I don't think it is the reason. Why? Because CMAltimeter can be used without CLLocationManager, and the issue is still present.
I have extra information though: 1 of the 2 test devices has the altimeter that works again. There is something new in Core Motion that somehow can block the absolute altitudes.