Is this "but which" here used correctly?

What is the part but which supposed to mean?

"At that time, his philosophy, which was concerned with productivity, but which was often misinterpreted as promoting worker interests at the expense of management, was in marked contrast to the prevailing industrial norms of worker exploitation."

I am confused about the but which part. If "which was often misinterpreted as promoting worker interests at the expense of management" part is a relative clause, then what kind of word is "but". Similarly, if "but" is to be a conjunction, then "which" should be "it", isn't it?

The sentence is correct. The whole of "which was concerned with productivity, but which was often misinterpreted as promoting worker interests at the expense of management" is the relative clause concerning "his philosophy".

It's the length that may be causing confusion. A simpler example would be:

"The door, which was thought to be locked, but which later turned out to be open, was at the other end of the room."