I’ve always thought that having two temperature attributes was redundant. It seems that WD has also taken that view, although there appears to be an inconsistency in their choice. AFAICT, “Airflow Temperature” and “HDA Temperature” use the same temperature sensor. In some models, from other manufacturers, attribute 190 is defined as “Temperature Difference from 100”, so it is directly related to attribute 194. This appears to be how WD has implemented it.
All other version of WD20EARS or other EARS Model have attribute 194. Only this version without it, and with some other unknow attribute. I am sure it have some problem within firmware.
I agree that it’s an annoying change, and that WD should have stuck with attribute 194. Unfortunately there is no real standard for SMART reporting, so manufacturers can do what they like. That makes it very tough for SMART software authors. In fact the smartmontools project maintains a database of drive models so that you can include such changes. You can also customise the reporting of each attribute in order to accommodate any variation or peculiarity. I have found the smartmontools forum to be very helpful in this regard.