If your channels are setup correctly (which i guess they are since 7.21.1 works) id assume its a bug.
https://help.mikrotik.com/servicedesk would be the place to go for that.
Or you could try 7.22beta.
on the AX devices so far i only remember USB being the culprit:
https://forum.mikrotik.com/t/chateau-pro-ax-wi-fi-radio-stops-completely-failed-to-set-country/