hello friend, i study forensics. its common for corpses to still have hair, as the same with bone it doesn't decomposed just yet reason being that is it composed of collagen and calcium which are durable proteins so for it to completely decompose it takes years like a lot. hair is made with keratin which is also a durable protein.
First, I shall thank you for clearing my misconception.
Next is the reason why I said that, I just thought that it's kinda weird. I know that hair could last quite long before decomposing (that is, in the right environment)
For this specific case tho, It's been 5 years since she got taken by the trash, assuming she managed to survive for a year after that(which is very unlikely), that would mean that she gone dead for about 4 years, I assume that's enough time for hair to decompose in such a damp place like a dungeon, at least some of it.