iPhone and iPad download gigabytes of data from gmail
I have an iPhone XS Max and iPad Pro 9.7 both running iOS 13.4.1. At various times, one or the other will download gigabytes of data from Gmail over the course of a day or so. I see this in the traffic monitor reports of my Synology router. The most recent report showed my iPhone downloaded over 80G over the last 24 hours. A few days ago, my iPad downloaded over 50G of data. Each device has two Gmail accounts set up as well as a couple of iCloud mail accounts. One Gmail account is a Google Apps account, the other is a normal Gmail account.
This has been happening intermittently for a few weeks and I think it's only occurred on iOS 13.x. I've rebooted both devices, removed and re-added the accounts, tried setting up only one account at a time on each device, and even reset both devices. The problem doesn't happen constantly; it appears to be random. I don't even have much data in either Gmail account (a few hundred megs at most). I don't see this with my Mac or my husband's iPhone 6s Plus or his iPad Pro 12.9 (2018).
Any idea what's going on?