FYI I got the following from support:
Thanks for reaching out to Zwift support!
I’m sorry that you have run into this issue, but we are here to help!
After a bit of research, I have found another member with this problem. Here’s what they had to say they learned by talking with NPE:
Currently if you want to have cadence running shown on Zwift , you can, but only if you also connect the Runn to the Zwift Companion app via BT at the same time.
2nd tip. Too many stickers or marks on the treadmill belt can be bad idea, as they can blur at high speed and cause issues. Keep your stickers at least 2ft apart.
Here is the forum post so you can see what else they have to say.
Give that a shot and let us know how it goes!