iCloud backup burning data, even though turned off

My son is living abroad for a while, so he has with him an old, unlocked iPhone 12, and is using a local (Ghana, Africa) SIM card and data plan. (His iPhone 15 with U.S. AT&T service is still usable, but he has cellular data entirely turned OFF. He mostly uses the 15, if at all, on wifi, and as a fallback in case of emergency.) He is logged into his same Apple ID for his (unused) iPhone 15 and his iPhone 12 with Ghana SIM card.

He has cellular data for iCloud backup turned OFF, because his data is limited. However, for some reason, every few nights, his phone still seems to burn through about 10GB of data for iCloud, quickly depleting his data allocation. He found an old discussion thread on Apple from last November that attributed this issue to an iOS 17.1 glitch, but he is running 17.5 now. And there were no clear answers for how to fix this issue -- just a number of frustrated users reporting the problem. The only solutions offered were trying to toggle icloud backup on and off, which he has tried, to no avail.

Anyone have any insights as to how to fix this thing? He isn't right around the corner, and it is essential that we preserve his data just so we can keep in basic touch with him. It has been frustrating and stressful, and we have no idea where to go from here.