Zen Master Badge

They were all recorded on my watch and then synced with the app.

Could you post a link to one of them? 

https://connect.garmin.com/modern/activity/4875058800

I hope that worked.

Yes, it looks all right. In fact, I just checked my records, and I did already 7 yoga sessions since the 28th April, and did not receive the badge either. It means there is either a bug, or some undocumented condition, like for example the mandatory use of PulseOx to measure the breathwork, or a minimal duration of the activity, or certain minimal drop of stress level during the session (all that is just a wild speculation). I tried manually adding poses, and respiration rates to all the yoga activites, to see whether it helps, but these ones did not make any effect.

I am afraid you'll need to contact the support to find it out. Let us know if you succeed.

OK, so I finally got the Zen Master badge after my today's yoga session. It was the 8th yoga activity since 28th April, and 4th since 1st May. So either you need 8 yoga activities, or it starts counting since 30th April, instead of 28th April, as stated on the badge.

I had the same exact problem and solution.  Today was my 8th yoga activity since April 28th and I finally was awarded the badge.

I woke up this morning and had my badge, I didn’t do any yoga yesterday so maybe that means they fixed whatever was wrong. I had a friend who hasn’t done yoga in a year and she got it a few days ago. Hahaha, there was clearly an issue.

I have over 10 Yoga session since May 1. However, these are all manual adds. How does one change the activity list to include Yoga? I have a VivoSport.

Manual activities do not work for badges. Have a look at you user manual here and select the "Other" activity type when recording a yoga session. Change the type then to Yoga in Garmin Connect.