Why your QR scans show as Unassigned in GA4
GA4 files traffic using a fixed list of recognised medium values, and there is no rule for a QR medium. The sessions are not lost, they are unclassified, and there are two honest ways to fix that.
Guides to QR codes that keep working
Guides on dynamic QR codes: how they work, why printed codes stop resolving, how a scan reaches the right GA4 channel, and what to check before you print.
Do QR codes expire, and what actually stops working
A printed QR code has no clock in it. What expires is the service behind it, which is a business decision rather than a technical one, and it is worth checking before a print run.
What a QR code costs over three years, and which part is not the subscription
The monthly fee is the small number. The one that decides the comparison is what a reprint costs when a code stops resolving, and that risk appears on nobody's pricing page.