Hey All, sorry for the trouble!
Border lines will typically only appear when exporting an image/frame/group/object that has fractional pixel sizing or is not sitting on whole pixels on your canvas.
FYI, i was having a similar issue: in Figma Slides, I was using a template that had really nice backgrounds consisting of an image, overlaid with a linear gradient to soften the image. In the Figma Slides web app, it looked fine; however, after exporting, there was a strange, miniscule, approx. 1px border around the edge of the background image. This border was due to the linear gradient layer being not precisely on top of the image, such that at the edge, it was just barely not covering the whole image. This for some reason did not show up on the Figma Slides web app, but did after downloading as PDF.
To fix this, I just expanded the linear gradient layer slightly over the edge the of the background image, and it fixed it.