If it’s a shower only issue, it sounds like it might be the tub spout. The diverter might not be closing fully and this could se sending the water back into the spout which could find it’s way back into the wall. I came across a product recently that’s called red disc seal or something and this might be a cheap attempt to fix the problem. My spout is kohler and it only has a set screw and it allows water to travel back down the copper pipe into the wall. I’ve been chatting with customer support a lot recently