I had this problem and saw another post in which people didn’t had their answer;
I tried everything i could, in my modest experience with Figma, but i kept getting the exports with a thin white line, sometimes top, bottom or sides.
I tried to increase the canvas with couple px, exporting multiple sizes (x1, x1.5., x2, etc) - had same results, nothing worked.
In the end, i managed to find the bug and the fix.
My solution is pretty simple:
Create a new frame with the desired size (exactly what you need, not +/- px). Then, you can copy-paste the graphics from the frame that has the export error to the newly created frame, just don’t copy-paste the whole frame. I noticed if you copy-paste / duplicate the frames is when the exported error happens, be it with one line top, bottom, two lines on the sides, etc. Double check in the preview area, and export.
I started having this problem when i was duplicating frames in order to change few items, such as titles and subtitles, and nothing else helped me.
Hope this will help, even though its been months since you posted.
Cheers,
Florin