It looks like perhaps the new carpeting didn't go all the way to the right edge of MapX0Y2, left edge of MapX1Y2 or both, and so the line you see is actually the original wooden floor showing through.

A good rule of thumb when working on mods that involve multiple maps is to lay out all the maps needed on one canvas, make your changes, then cut the maps back into 512px x 512px parts. It worked well for me. I'm sure you'll find a way that works well for you. Nice job VelvetVamp!