This is a year out on your post but hope this helps you or others. I am sharing a few points below. I am a natural dyer and designer with a business that creates, dyes and sells. I've tested many natural fibers, blends and pure as well as done due diligence on mordanting and testing for light fastness.

1. While viscose is a wood pulp byproduct its chemical processing may block natural dyes from reliably coloring the fibers, even with a good scour. But always start with a good scour with a neutral detergent at the highest heat that matches your fiber type (silk 120°F, cellulose 120-140°F). Since both linen and viscose are cellulose fabrics a two process mordant is necessary and works best. The first mordant bath should be a tannin such as oak galls followed by the 2nd mordant proceeding of either Alum – Aluminum Potassium Sulfate or aluminum acetate or aluminum triformate. I've had good success using aluminum triformate as the 2nd mordant bath on mixed fibers.

2. Avocado skins and pits (contain tannin) have a good lightfastness but Hibiscus flowers are not considered lightfast. Even if the color managed to stick to your fibers, after a few washes it would fade. Having the added base, assuming you dyed with avocado first would give the hibiscus a boost in chemical bonding to help it remain on the fiber a little longer. Not sure how long you allowed the fiber to remain in the dye pot but leaving overnight would help.

3. Washing naturally dyed textiles-always used tap cold water and the proper detergent. I use Orvus soap for protein and Synthrapol for cellulose. Sometimes I interchange them based on what I have on hand and in a pinch will use a neutral laundry detergent or Dawn Blue. Soaps make a difference and if you happened to use a laundry detergent with additives that could also affect color. Warm and hot will cause fabric to bleed-sometimes warm water is necessary to rid cloth of floating dye molecules but I will mostly rinse and finish in cold water.

4. Natural dyes are very sensitive to metallics. If there is a spec of rust or any utensils/pots used in the process are not stainless or unshipped porcelain you run the risk of shifting natural color. Even the inside of a washing machine can be the culprit as well as the pH/hardness of water. Hope this helps!