When you say "moving mask", could you describe what you're doing?
Putting a mask on a layer should have no problems, but the Set Matte effect would cause issues if the scales were different. Creating a solid layer, masking it, and using it as a track matte should also work.
Screenshots of your full interface would really help us to see your problem more clearly.
By moving mask, I am using set matte on the composition to make the alpha channel matte a rectangle that goes down the screen. Both the blue rectangle, the mask, and the composition, have scale 100%.
End of animation:
The intent was to have the entire imported composition be displayed in front of the blue rectangle, which also serves as the mask.
The Set Matte effect is the issue. It currently only works if both layers are the same size. (Join me in filing feature requests to improve its functionality!)
Duplicate the layer (using the copy with property links option if you want to make sure any changes you do to one are reflected in the other) and use one as a Track Matte instead of using the Set Matte effect.
A few other ideas. I am with my phone so can't test all of it or show how it's done but:
1. What if you set the blue rectangle as vector solid by toggling it's continuously rasterize switch in the switches panel? Does set matte behave then?
2. same as option 1 but with a rectangle shape layer (which you can apply a feathered mask if you want)
3. If these are the only 2 layers in the composition then lose the set matte and you can use preserve underline transparency switch which is located in the modes panel for the logo. This is Ae's Photoshop's clipping mask.
Thanks Szalam! Your solution worked. I will definitely let Adobe know in a feature request.
Glad it worked!