I want to mask out an object from a clip. Because the object rotates (and I don’t know how to rotate the rather complex mask shape I created with the pen tool) I decided to apply the mask to an Adjustment layer (which I can rotate) and the result was good. But… the mask doesn’t mask. Shouldn’t it?
You can't mask out layers below with an adjustment layer. The mask will affect any effects you have on an adjustment layer, but it won't "remove" the areas in the layers below. Instead of putting your mask on an adjustment layer, put it on a solid and set your layer below to Alpha Matte Inverted.