Could it just be dosage? Commonly found dosage is 10-20x what you'd actually secrete and IIRC absorption is actually quite good.

My understanding is that melatonin can overload certain receptors and cause an issue with fogginess since it works directly on the brain. 5HTP is more broad and is “gated“ on the same metabolic pathway, so cannot overload the system the way melatonin does.