Colors are becoming dull in illustrator for no reason

Something odd is happening to the colors. I have not made any changes to the document or settings. However the colors (global spot color pallete) of the document have changed after I logged off. Next morning opening the same file and they became dull. (see pictures). 

If I copy paste 'bright color pallette' into the 'dull one' document it also becomes dull

Luckily I had a backup of the same file, and there the colors are not 'ruined'

Why is it happening and how to solve this.... is it a bug?