Apple watch pausing during workout with external HRM

I have recently purchased a reconditioned Apple Watch Series 8. I also use a Garmin HRM Pro plus.

I do Crossfit and wear wrist supports, it is therefore impossible to wear the watch while doing a workout.

I have connected the Strap to the watch using Bluetooth. I have checked the watch receives the heart rate data from the strap and prioritises it over its own heart rate data.

The problem is that during a workout the workout app keeps pausing. I have turned off auto-pause. I have turned off wrist detect. I am at a loss of what more I can do. I also thought maybe I had moved out of range (although well within the published Bluetooth range), but testing it at home, it again keeps pausing when I am right next to the watch with the strap on.

Everything is up to date and in the week since I got I have already had to do a complete unload/download on the watch on the phone.

In desperation I have tried to go via the Garmin Connect app and/or Strava app, but I think you can only connect the strap to one thing at a time, even though in theory it can manage up to 3 bluetooth connections.

It is very frustrating to get to the end of the workout and find you have only the first 5 mins of data.

It is also frustrating when your tech is insistent that it knows better than you want you want to do/are doing.

Any suggestions that do not include things I have already tried would be most appreciated.