Digital BW, The Print

I have a series of prints made with MIS VM-Inks with Paul's neutral curves
on BrigthCube SemiMatte,spayed with Krylon. Two identical sets,one was
matted,framed behind glass and hanged on my walls. The other went to an
acid-free album.
Funny enough,the hanged ones remain with their original beautiful blacks and
original contrast. The one in the album warmed to a horrible brown cast and
lost contrast.
Any thoughts? If I had to bet on one set to show that bad
behavior,that shoud have been the hanged one,which is exposed to ligth,isn\ufffdt
it?
Best,
Andr\ufffd