this is happening to a lot of my animated gifs too, but unlike you i cannot for the life of me figure out why. i thought when i stumbled across this post i'd find my answer, but alas, this 1 pixel interference remains!!! mine is occurring across all frames however, and like you i can't see it in preview mode - only on export or preview in a browser.

i've seen the line on gifs i've made in the past (never actually finding out WHY, it seems to be random), but this gif is only a few frames. it's composed of a few rectangles where text slides in from the side. it's completely basic and there's no reason for this line. a few times it looked like the frame behind it was showing, and i made sure there was absolutely NOTHING behind it in the frame and my background rectangle overlaps the ENTIRE image, showing no white space. but still i get this 1 pixel annoyance. no layer is the same size nor in the same place. there are no duplicate shapes on a single layer.

what is it??? these gifs are tiles for corporate companies, and they don't look very professional with a line running down the side.