This is really faulty logic. Your emotions moving in cycles is not in any way proof of some purely biological issue. Non biological stimuli like external stressors also come and go.
"Recent findings in the field of neuroscience have, however, demonstrated that a single functional circuit, incorporated inside the broader dopaminergic mesocorticolimbic system, seems to be involved in the various experiences of pleasure"
https://www.frontiersin.org/articles/10.3389/fnhum.2018.0035...
Going back to your accusation of "faulty logic": an interruption in the ability to experience pleasure, which is established to be controlled by biological processes, having a basis in a biological process would be deductively valid logic. Anhedonia is an inability to experience pleasure.
Well of course on a certain level your ability to feel anything is the result of biological processes, but that doesn't mean that the specific things you feel aren't a result of the specific experiences you've had.
Nobody in this thread is arguing that feelings happen completely independently of biological processes, but there's a hugely important difference between "I had a bad experience that altered my feelings, and that's reflected in my biology" vs "my biology just did something of its own accord that had nothing to do with my life circumstances, and that change altered the way I feel".
You seem to be arguing that because anhedonia correlates with certain biological changes, that it must be caused by those changes. That could be true (and probably is true in some cases) but it does not necessarily follow that it is always true, or even true the majority of the time; the causation could could flow in a different direction.