This has been happening to me for a while as well in New Zealand on 2degrees network. iPhone X, 16.4.1
Last night accidently left mobile data on and Software Updates took 485 MB.
I've turned off everything I can find as per everyone else.
For me the only new data point is this happened with iphone NOT plugged in to charger. I do have an apple watch and wondered if it might be due to that taking updates, but sounds like that wasn't the issue.
The downloading happened at night since I monitor mobile data use very carefully now.
Software updates through Settings->General->Software Update definitely isn't 'seeing' updates with mobile data off, so have to assume it's a bug. Making the phone pretty much useless except for messages and telephone since I have to keep mobile data off all the time now unless I'm careful.
Edited to add that I don't think the phone has ever been enrolled in any beta, though many years ago my iphone 6 had used the trick of the AppleTV developer profile to prevent iOS updates over wifi, and my X is the direct transfer from the older 6.
I'm hoping this data leakage isn't some weird AppleTV ghost software updating itself.