Is there a mask around the image you're trying to corner pin?
That might be causing the issue. Corner pin operates assuming you're working with image dimensions that are the same size as the comp. You can get around this by precomping the layer and fitting the layer to the size of the comp.
Again, it looks like you're applying the mask to the corner pinned layer. Adding a mask to a layer that is corner pinned will change the dimensions of the layer and therefore change the way the corner pin is applied.
Go like this:
1. Precomp and expand the insert layer inside the comp
2. Apply the corner pin to the precomp
A) Precomp the corner pinned layer to maintain the dimensions and apply the mask OR
B) apply the mask to a new solid and use it as a track matte.
Martin Brennand | Product Manager | Mocha - Boris FX (Imagineer Systems)