Wish I knew what to tell you on this issue. But I will ask the manufacturer and print of the fabric, so I avoid it.
There are others who have had the problem you are having and many suggestions were made. Maybe you could do a search on the board for them?
I am hesitant to suggest this, but will - how about putting some borax in the washer while it is filling and washing the quilt one more time? Borax is a whitener; I use it occasionally, but not on quilts, thus the hesitation about the suggestion.