Every paper reacts differently to UV light.
Mat lines are caused by the open cut or bevel of the mat. The correct description for map you described is mat line with light staining. I strongly recommend not using the commercial deacidification spray. You will not find a museum conservation lab using these products as they have not passed the test of time. As this does not happen overnight many people ignore the warnings it looks good today but these agents stay within the paper fibers eating away at the starch that holds the fibers together eventually the paper begins to get soft.