My guess would be there is something going on behind the tiles along that horizontal line that is discoloring the tiles…maybe from the heat of a hot copper water pipe?? Or an issue with electrical wiring that could have overheated? Or some other product like glue or a piece of wood that has chemically interacted with the tiles along that line. Is there a way to make a small access hole on the other side of the wall to see what’s going on there? I also notice some cracks in the grout in that area and if it’s an old bathroom, there maybe be mold behind there. 🤨

I like the water pipe idea. A thermal picture would be interesting or failing that run the hot water and feel if the tile gets warm in the shadowy areas. It could be a wall board joint, but the lower one is definitely sloped. They might also be cold spots where condensation has formed over the years

Thank you! I would have never even considered that. Truly appreciate it!