What is the interpretation of semen descending with urine after waking up from sleep, without seeing any trace of it on the clothes, without feeling nocturnal emission, and without desire?

What the questioner presumes to be maniyy (sperm/seminal fluid) is actually wadyy, which is a fluid similar to maniyy in consistency but is discharged without sexual arousal and typically appears after urination. Even if the discharge were maniyy, it would only necessitate wudu' (ablution) if expelled without sexual arousal, and this is the view of the majority of scholars. The explanation for this phenomenon should be sought from medical professionals, as it varies depending on one's bodily constitution and health condition.