Assuming it is legit, it could be some fibre/ash, or, the dried fruiting body itself. It will look darker than the fresh mushroom when dried (that part is normal). I don't think you should expect it to be totally water-soluble either depending on the extraction method.

Most mushrooms are composed of around 90% water by weight. The remaining 10% consists of 10–40% protein, 2–8% fat, 3–28% carbohydrate, 3–32% fiber, 8–10% ash, and some vitamins and minerals, with potassium, calcium, phosphorus, magnesium, selenium, iron, zinc, and copper accounting for most of the mineral content
https://www.ncbi.nlm.nih.gov/books/NBK92757/#_ch9_sec5_

You should share the brand if it is well known because there'd be other commenters that can confirm what you should expect.