It is hard to say for sure. Can you provide a screenshot?
The most likely case is that the texture is filtered to smooth edges. If you select the image in the File System tab of the Godot editor, and then look under the Import tab, there should be a checkbox labeled Filter in the import properties. Uncheck it and click the (re)import button so the settings are applied. That should disable the any texture filtering, which may make the maroon color line disappear.
Also, the maroon-colored line could be part of the source image itself. It might not hurt to double check the image to see if there are maroon lines in the image itself, just to be sure.
(Side note: Welcome to the forums!)
