I'm assuming this must be an error on my part that has something to do with the settings.
In this composition, a white square (with the triangle on it) comes across the screen from the right side, revealing a green background gradient. The problem? Whenever I add position keyframes to the white square, pixels of blank space appear to its right on top of the gradient once it stops. The blank pixels are not there as it moves across the screen. If I make the white square invisible, it's clear that the blank pixels are not a part of the gradient. If I place any other large solid behind the white square, the blank pixels are still there.
Additionally, the pixels stay blank in exports. I just upgraded After Effects, but the problem was happening in CC 2018 as well.