I purchased my Xiaomi Mi A2 directly from the Mi UK online store in early March 2019. On powering, it updated to Android 9, it has worked flawlessly with an average full charge lasting 2 days. I had heard reports the proximity sensor had issues; I ran a full hardware diagnostic and the proximity sensor passed multiple times without error. My phone is issue free with Android 9, I would suggest you perform a factory reset and let it automatically update to Android 9, then go into Settings > System > Backup > Back up now - this will backup your phone to your gmail.com account in the future.
AndroidOne is brilliant!
Hope this is helpful.
Barry Sumner