Firmware downgrades don't always work on the Boox. Some versions permit downgrades, others don't. This issue has happened before with 1.9 / 1.8 and 2.1 / 2.0, when people couldn't downgrade even if they had the older firmware. It may well be that 2.2 can't be downgraded to 2.1.
If you're willing to take the risk, the firmware for 2.1.2 appears to be online even now. See
http://web.archive.org/web/201909181...ds/#max-series for links to Google Drive folders with the firmware. Good luck!