My heart rate monitor typically doesn’t register on any device I’ve used (including my Garmin 520) until I’ve warmed up for a few minutes. It’s particularly an issue when I work out inside in the winter on a trainer.
This normally isn’t a big issue as my Garmin will just pick up my heart rate monitor shortly after I begin my ride. My problem with Zwift is that Zwift doesn’t seem to try to pair with my heart rate monitor once I’ve left the pairing screen. So, I go the whole ride with no heart rate and as soon as I’m done with my ride and go back to the menu, I see that it’s now reconnected.