Hm I see. You have a point that the causality is not necessarily how I described. But I am not convinced it is the opposite.
What causes the workplace to make you have high adrenaline and cortisol for too long? Stress is perceived by the body as a threat and it subsidizes when the body perceives that the threat is overcome. If we exert ourselves for a while but then we have a good outcome and we celebrate with our colleagues, we will have dopamine and oxytocin levels rise and adrenaline and cortisol subsidize. Something one might say makes the exertion meaningful.
So I hear what you're saying but I think we're both describing the same thing from two perspectives and not actually different causality directions. I described the subjective interpretation and you described the biochemical process underlying it.
Burnout is caused by your body running on adrenaline and cortisol for too long as you’re pushing past what is sustainable. Eventually your body says - enough! And it forces you to stop.
The _symptom_ of burnout are as you describe: essentially you no long see the point of what you’re doing.
Speculatively, I believe the drop in motivation is your body’s way of stopping you pushing it any further. It’s a defence mechanism.