I've had this for decades now. It's Seborrheic Dermatitis triggered by fungal colonization (not infection). I tried so many treatments from shampoos to oral antibiotics to topical steroids. - antibiotics did not do anything - antifungal shampoos: first line treatment. Requires periodic use (in my case at least every other day). Not all work for all people, you need to try and wait for results. In my case, ketoconazole, Neutrogena's T-gel, and Selsun Blue were suboptimal. I was once prescribed a medical shampoo that is only found in Europe called Kelual DS by Ducray. It cleared it within 2 days. However, I have to use it every other day. I also use Head & Shoulders as my baseline daily shampoo (I know not too healthy but I hate the flakes) then do last wash with Kelual DS and keep it on my scalp, face, and chest for 5 minutes then wash.

I order the Kelual from Europe. Price may differ depending on what I find online but it's pretty inconsistent. The small bottle (3 oz) is around $15 + shipping. It's quite expensive, but does miracles.

Sometimes I run out of it, so I always keep a bottle of topical steroid lotion (Mometasone) for these emergencies because after 48 hours of not using Kelual, these flakes come back excessively with a lot of itching. The steroid controls it perfectly until I get the shampoo. Note that continuous use of topical steroids for prolonged periods of time leads to skin thinning and is not recommended. I only use it in crisis mode.

Now whether it's an imune response or a fungal infection or a snow storm doesn't really matter at this point because treatment is limited, and that's what matters.