Workouts are only added to your Milestone journey after the Results screen.
For the workout time and class tally to be added to your Milestone, you need to reach the Results screen and then continue past it - either back to the Home View, or to your Cool Down.
If you remove and power down your headset before progressing past the Results screen, your workout will not be added to your Milestone.
Any trouble at all with this, please do reach out to our Support team, and we'll be happy to help.