The best explanation I've seen thus far, assuming there aren't other complicating factors (which there often are) is that the immune system stays hyperactive in some people after infection. This causes excess production of IL-6 and IL-10, which in turn causes something called the kynurenine shunt. That in turn results in lowered central serotonin rates in the brain and heightened glutamate. This off balance results in lowered dopamine as the production of serotonin and dopamine are comingled. Dopamine usually breaks down to your typical epinephrine and norepinephrine levels. But since dopamine is at a lowered state, you get less epinephrine and norepinephrine. I'm guessing, but do not know the pathway, that lowered epinephrine and norepinephrine explain the lowered cortisol.

Either way, the treatment should be the same. Take a corticosteroid to lower the IL6 & 10 levels, ensure the patient is getting proper sleep and nutrients, and not doing anything to exacerbate lowered serotonin/dopamine levels, and cross your fingers.

I've started to hear of companies that are testing for elevated cytokines as a proof of long covid, and my family member's hospital has started educating their staff on cytokine release syndrome in the rare patient who has a reaction to a vaccine shot, both of which, for me, imply the industry might be moving in this direction for explanation. fingers crossed