Hi there, Click with right button on the file tempdecal.wad and mark the checkbox "Read-Only", so, the game can't write this file again. This happens when you load a second map.
Don't forget to paste your custom tempdecal.wad again, then you mark the checkbox.
You are using what program to make your sprays?? I do recommend the Wally, the best program to create textures, decals and sprays.