If you went this far without success, you might as well reinstall the OS, because a factory reset in the Android world only deletes user data and Apps installed by the user and their data, the OS itself is left untouched …
In-place upgrades to a new major OS version (as opposed to cleanly installing the new version from scratch) can cause weird behaviour of all sorts. This has nothing to do with any particular OS or device, it can happen with all of them, software in general became just hopelessly complex.
Vendors can’t test every state an installation might be in, and they are not bold enough to leave cleanly installing from scratch as the only upgrade option, because obviously many users wouldn’t like that. So they do what they can with testing and roll out in-place upgrades keeping their fingers crossed.