I've got my smartphone set to automatically sync to my NAS. I run a cheap Chinese Android phone (Oppo A80 5G I think) which is tied to Google. Or a google account anyway. I did have it set to sync/backup over WiFi only, but Google seems to want to fight with my Synology Drive app (the Synology equivalent of Google drive) over the task. Google seems to try and take over the backup, or at the least stop the Synology app from working. So I've set the backup settings to all the time -not just WiFi, and we'll see what happens. It might also be happening when the phone gets its updated too, we shall see.
It's actually pretty annoying, one of the reasons I've switched over to Linux, as Microsoft Windows has made it really really difficult -nearly impossible, to try and store your own data on your own computer. Google is not far behind. It's just money grubbing, trying to get you to pay for their storage, rather than use your own. And they're catching tonnes of unsuspecting people too by the way.