Question abour red dye (clothing, not food)?

It is all about colour fastness.

Colour fastness is: "That property of a pigment or dye, or the leather, cloth, paper, ink, etc., containing the coloring matter, to retain its original hue, especially without fading, running, or changing when wetted, washed, cleaned; or stored under normal conditions when exposed to light, heat, or other influences."

The chemistry of what makes dyes fast is complex and involves the type of material, actual dye and use of mordants. Unfortunately you don't tell us what material your sweater is made of, or where is was actually made. The most likely answer is it was made in another country and costs were cut using a cheaper red dye that is not colour fast.

Having said that, red dyes are notorious for this; everyone [?] knows you show never put red items in a white wash or you'll get pink whites! Red is a particularly difficult colour to get as good colour fast dyes.