It's a common issue in games due to the way textures get rendered and filtering. In short, to solve it, the most reliable way is to have your tileset have each texture separated with a fringe of extra pixels around the tile.
Otherwise, you can set your project to use pixel snap (in the Project Settings), which will limit the smoothness of the camera scrolling but force all textures + the camera to snap to pixel locations.
For more background on the issue: it's due to the tiles, which are individual squares, not falling on pixel-perfect positions depending on where the camera is or where you place the tileset. Your camera can fall between two pixels, so the renderer has to sample on the edge of your texture, causing some of these artifacts.
Thank you. I decided to use "pixel snap". Fortunately, I am not seeing any performance issues.
I hope Godot fixes the issue with a line artifacts because having to add a fringe pixel seems like a bit of a work around.
It's not specific to Godot, this happens in ~all game engines. Again, this is due to what a tilemap is.
And the pixel snap doesn't reduce performances, just the smoothness of the scroll.