Draw Mask is cutting off parts of the picture it shouldn't be. How can I fix it?

I'm making some fun videos for friends involving a split screen effect, and I'm using Draw Mask to crop an upper layer onto the lower layer. However, there seems to be a horizontal line that it has added, where it cuts off what's below as well. I'm attaching a picture for reference. To add, I think I can confirm it is Draw Mask doing this because when I "invert mask" it shows me the lower part of the picture. Is there some setting I have on? I can't seem to understand how to fix this. Thank you!