Here you can find both the releses: http://gawisp.com/perry/fenix_D2_tactix/ Comparing the folders, it seems that 12.51 is really the latest because it includes newest version of Sensor_hub and Text. Since it seems that from official beta link you download 12.51 (see answers in the beta release topic) even if the page reports 12.21, I think they have only made an error writing the page. The only strange think yet is the date of the release files: the 12.21 it seems really to be generated after the 12.51, but it could simply be 'loaded' after due to a lapse.
Garmin has rolled back the beta software version to include the changes below. This version addresses a higher than expected battery drain for some devices. This version does not include the changes in 12.51. If you are already on 12.51, you will not need to back date to 12.21 unless you are experiencing high battery drain.
Important: Back dating from 12.51 to 12.21 will trigger a factory reset on your fenix.
Changes made from version 12.00 to 12.21:
Fixed an intermittent issue where the device would display an incorrect sensor error message.
Fixed an issue where some devices were reporting a higher than expected battery drain.
