I can confirm the solution of using a full factory reset to get a Garmin Dual HRM to connect properly. In my case I’m using an iPad pro 10.5 inch from 2017, running 13.5.1 before and after the reset. At the moment I’ve installed Zwift as the only additional app apart from the general Apple pre-loads and the Garmin dual HRM is now happily showing heart rate again.
I’ll start adding apps back on the iPad and give an update if I find one that kills the connection.