2 ways to get this "bug" to happen,
1 you have a launcher that is fucking things up,
2 you are in dev mode and not logged in so your UUID is going to change
show us proof that this happens and report it to mojang/forge dont "fix" this on your own you will break more stuff on the way
https://github.com/BenjiBrat/MinecraftModdingError/blob/cee4f1a8b06ef0c84a1433d6f001dbe08f01baa7/src/main/java/com/benjibrat/minecraftplusmod/entity/EntityWatcher.java#L138
if(1 == 1) ??
on github you have the MCP folder remove that