In AE 25 i found bug, which is appear at the moment you use compositing for effects on layers using masks. Then, when you add another effect to the chain, something happens and the mask for effect sometimes works like without effect compositing. Sometimes there is only one pixel column on alpha channel.
I am ataching screenshot and project file.
When you change curve (no matter which one), the pixel noise dissapear and if you will change the other curve, it appears back. Try to play with it..
It is unreliable for now use compositing effects through mask on one layer in chain with other effects, I saw similar strange behavior for last few days..