Wrong Tax rate/area in contract billing detail after enabling alternate tax/rate

Question:

When entering a contract billing line via P5202, the tax rate/area is taken from the contract master.

However, after enabling the alternate tax rate/area functionality, the tax rate/area is taken from the customer master.

We did not set up anything yet in the alternate tax rate/area file (P40081), so it should fall back to the default.

How come that after enabling the alternate tax rate/area functionality, the logic for retrieving the default tax rate/area changes from contract master to customer master?