The old Glock "Tennifer" finish was pretty darn resistant to all forms of corrosion. It was also, as I understand it, incredibly toxic to produce with severe health ramifications to the workers.
I can understand if the corrosion resistance of the finish diminishes because they can no longer essentially kill people to produce it.
The QPQ-based nitrocarburizing processes that are used most in the US seem to be heavily dependent upon proper preparation of the parts that are subjected to it in batches, and sometimes that seems to not happen on some batches or on all parts in a batch. As best I know most companies are outsourcing that work to subcontractors.