Traffic acquisition unassigned

I’d like to better understand what leads to UNASSIGNED value on Traffic acquisition: Session primary channel group, while the same sessions have source/medium (see attachment).

Compared to gtag implementation with gtm i’ve reduced a lot the number of unassigned, i know they’ll never be zero, but i’d like to know the technical reason of this situation.

Thank you!

I think this article will help you: Unassigned and not set traffic source in GA4 - Stape

Thank you.
I’ve already read that, my configuration seems to respect what said there, sessions unassigned show direct/none as source/medium, all other reports are fine. In server side client ga4 is set to use server managed as client and user identifier since i’m using a custom subdomain to serve gtm. Maybe i need to investigate on fpid, but i assumed it should be fine without modifications due to mantioned config. User acquisition for istance is fine: see screenshot