Following persistent recent problems with Android Auto, a factory reset was called for.
Despite backing up the phone as instructed with a forced Google backup (both accounts I had on the phone) when the phone restarted, neither account had heard of any backups. Even though it confirmed them before the process.
Luckily being a cynical type, I had a Samsung SmartSwitch backup. But that still mean signing into every single f****ng app again.
I also lost about 30 Google Authenticator accounts. (Again) luckily I had screenshots of a lot of the QR codes.
The average user would have been ***ed.