Interesting thread! I remember coming onboard as an excited newbie after I learned about Frixxon pens at a quilt show. Then I found out they left a white line on my dark fabrics after washing. Now I see someone posted those wash out -- thank you for experimenting. I guess I would have to wash again.
The question about what chemicals are in there which might cause the reappearing line, whether on dark fabric or put in the freezer, is a good one.
In any case, I used a pencil for the markings on my last quilt. Someone had shared a thread about all the stubby pencils her hubby uses to mark. I rubbed it right off without even washing it.
Someone asked, "What is the point of using them?" I guess the point is the point. You can make a precise mark.
My quilts will never go in a show or to a customer. I just sew for myself but I only use the Frixxion pen on the back of the fabric "just in case".