Android backup is lacklustre and as much as I'm not a fan of iOS, its backup and restore system is superior to Google's in my experience.
I've had to factory reset two devices recently, a OnePlus 3T and Pixel 2, so I removed my security and Google Account first, to avoid Factory Reset Protection, and then reset via Android's settings menu and when I checked my Drive afterwards, the associated device backup was also deleted, without any warning. I first noticed this with my 3T, when it only became apparent after I set it up as a new device and there was no backup to use for restoring it. After I did this with my Pixel 2, I checked my Drive straight away and its backup that was there minutes before was gone.
I never experienced this issue with my Nexus 5 and its backup, so I don't know why Android behaves this way now, but it's annoying.
What's the point in a backup system if it deletes the backup when you follow the correct steps to reset the device.