Vegas - Weird line when masking, picture included

Hello,

I was just trying out some stuff with my cat and I made it look like he was walking by using a mask.
But now it shows this really odd and ugly line in between the two parts...

I color corrected both tracks so they would match each other. I figured out that the line disappears when I remove the color correction.
But I need this color correction to make it look right... any way to get rid of this line?

Thank you so much

Tom

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