Packaged Bias Trim
I sometimes use packaged bias trim - I think Wright's is still the brand name - just bought some last year at WalMart to use for a touch of color on pillow cases -
The package said it was a cotton/poly blend, so I did NOT expect it to shrink. It did.
I measured the trim before and after it's soak - just to see what it would do.
I put the trim in a pan of hot water and let it set for about half an hour and then laid it out on a towel to dry. It shrank about an inch per yard. Enough to cause a pillow case to pucker which bothers me. I am very careful about not stretching or pulling the trim as I sew it on/in the item, so that would NOT be the reason for puckering later.
The shrinkage may be a contributing cause for why some bibs do not lay flat after they are washed.