With this little detour it works great for me
Hi, you should check out Navigr8. This is a new tracker app for Apple Watch and iPhone. The pro version allows you to connect all sorts of Bluetooth sensors (HR, Cad, Spd, Power) to your watch or phone and share the data between them. If you want to transmit heart rate data from your watch’s built in heart rate sensor to your Garmin you can do this via the phone app, which can be set up to look like a Bluetooth sensor. You simply connect to the Navigr8 heart rate monitor just like you would a regular heart rate sensor. Works a charm!
BlueHeart is free a software solution which uses your iPhone as a bridge between the watch and the Garmin, no other hardware required, and no subscription cost like Navigr8