I use a Garmin-Hrm Dual.
Pairing with multiple devices has never been a problem.
The first months that I used Zwift I already had the Garmin in combination with the Headwind and this gave no problems.
There was always a regastration of the heart rate.
I think this problem started after using Direct Connect.
However, after disconnecting the cables, so everything via bluetooth, the problem remains.
Sometimes I see a heartbeat, but when I go to a workout, the connection with the heart rate monitor disappears.