A strange bug with copying mask keyframes

Suppose we have a clip in which we want to synchronize mask animation of 2 different effects. We create the animation using keyframes and if we wanna transfer these keyframes to a mask in the other effect applied to the clip, Premiere Pro won't let us do that - you will not see the COPY in the right mouse button menu neither the Ctrl+C will work.

But if you CREATE a mask in that other effect and then click your mouse to the part of the window where the keyframes are placed, the COPY menu will appear, so you can copy the mask animation and paste it into the other effect. And in this case the effect just will get 2 masks - the one you'd created to make it all work and the other one - which you pasted (so the unnecessary mask can be deleted). See the video