Mentor will tell you this is by design. Take a look at https://supportnet.mentor.com/portal?do=reference.technote&id=MG588516&lang=en&prod=C107-S123-G192-P10287
Per that link:
According to the documentation " Manufacturing Inputs and Outputs Guide “, Pads are removed on signal layers only. Pin & Via pads are not to be removed from positive or negative plane layers, hence the application (ODB++ Generator) is functioning as designed.
There are a couple of Mentor Ideas that have been submitted, please go out and vote for these
Non-Functional Pads on Plane Layers removal from ODB++ - D7759