eero uploads tons of data on behalf of devices

Eero support has not been helpful so I'm asking here.

Every a few days, my iPhone may experience very slow connection speed on my Eero network. Upon checking, it had been uploading tons of data at full speed, as per the Eero app.

I am pretty sure no app on the iPhone (including backups and iCloud syncs) would have used that much data. The same issue happened to another iPhone and a MacBook Air in the household. However, by using the on-device network traffic monitor I couldn't see the uploads. But the uploads are reflected in both Eero app and the ISP's dashboard. And, for the legitimate uploads by real usage, they wouldn't have blocked the network almost entirely even at full speed.

I searched and looks like this is a common issue with eero (e.g. this thread). If I disconnect my iPhone from eero and use cellular data, then the connectivity recovers immediately. This  issue did not happen while the devices are on other Wi-Fi network, or on cellular. While it's doing uploads, only that device became very slow in connection, and other devices sharing the same network doesn't seem to be impacted a lot.

I have been observing the issue for over a year. It may have begun earlier than that.

The topology of my home network is:

Modem -> Gateway eero -> switch 1 -> switch 2 -> eero 2

                           |-> eero 3

                           |-> eero 4

Switch 1 is TP-Link TL-SG108, switch 2 is Netgear GS105v5. The modem is ARRIS TG4482. It is on bridge mode. One of the eeros that connects to switch 1 seems to be more problematic, although it does happen on every other eero as well.