Apple Watch 5 as HR monitor

In the past, I have used an older Apple Watch 3 as a HR monitor on Zwift, but I felt that it was too ‘steppy.’ That is, the HR graph was rather discrete, rather than smooth. I’ve recently upgraded my watch to a series 5, so I thought I’d check to see if things were better. (Even if things aren’t better, I don’t know that I care any more, and it would be nice to ditch the extra HR monitor.)

I can’t get it to work. I’m using ZCA on an iPhone 7 as a bluetooth bridge to the Zwift app running on Windows 10. I get the option of the Apple Watch on the pairing screen, and once I select it, the ZCA app on the watch reports, ‘Selected’ in yellow. However, the ZCA app on the watch never displays a heart rate, and eventually I see ‘No Signal’ in red over the Apple Watch selection on the pairing screen on the PC.

I remember having some trouble getting this setup to work with the old Apple Watch, but not this much trouble. Is there a trick I’m missing?