Microsoft's Band 2, the fitness watch, got a new firmware update earlier today. According to Windows Blog Italia, Version 2.3.21004 had patch notes indicating that the update implemented the already added Heart Rate Zone tracking. Seeing how we already had the feature, it's likely this update has more to do with bug fixes or improvements instead.
You can't access the firmware update until you make sure the app is updated as well. Former Microsoft Health app now named Microsoft Band was also updated with "improvements for devices upgraded to Redstone 2."
Perhaps the firmware update might also be related to Redstone 2? Make sure to update your app (if it doesn't automatically) and let us know if you notice anything different.