umm, i think its pretty much common that poly will do that to any wood, at least the high-school kid working at home-depot knew it, for a true colorless clear coat try polycrylic clear or satin buy minwax, thats what i used on my Pi as i wanted to keep the blonde of the baltic birch.
A Finished pic, i finished them late september and they haven't changed one shade.
I noticed the change of the poly on my unstained rack mount the very day i finished it. I guess it would do the same to a painted finish too.