Yes, this is indeed a valid bug and is caused by wpa_supplicant upgrade from 2.09 to 2.10. Built manually the older version and installed that on f5121 (XperiaX) and tethering does seem to resume working as it should. I wonder if this is an issue with the older kernel the device has or some configuration option that would be needed for the wpa_supplicant.
So now we know what causes this and we’ll try to address the issue.