Good news:
Since typing my first reply, I have received an email from Zwift support stating the following:
“After reviewing this case again, I went deeper and realized we are currently facing a known issue where treadmills are not pairing, and not appearing in the pairing screen.
I am sorry for any inconvenience this has caused you. We are currently working to get this sorted out. It has been fixed in 1.33.2, which will release soon.”
Hope that helps everyone.