I have had the same issues as others have noted on here.
I borrowed a friend’s Wahoo Tickr HRM and it worked straightaway on my Zwift ride. They are quite well priced so have ordered one and will use the Tickr HRM when using Zwift and the Garmin HRM when riding outdoors!
The Garmin Dual HRM works great with my Garmin 530.
Not a perfect solution, but needed to get heart rate showing on Zwift ahead of races in the Autumn and Winter!