Wahoo tickr run cadence erratic

Hey all …
I’ve got a wahoo tickr run that I am trying to get working for my daughter in Zwift for use with her treadmill.
I use the hrm all the time for cycling and not had any issues with it at all to date … however I never run, so that aspect has not been used much, if at all.
So anyway, the tickr run pairs with smart watches, head units for cycling and every laptop or mac, all the apps and phones we have work with it, both on Bluetooth and Ant+.
However after I fire up Zwift, pair it with no trouble and then successfully run through the calibration for cadence it goes a bit wonky in zwift … so if I fire up the running track, suddenly it jumps from 8.8kmh to zero, then back up to 8.8kmh, or 18kmh and then back down to zero and I stand for a second or two and so on… and all the while I will be doing a set 8 kmh on the treadmill, hr is reported solidly.
I tried zwift on a mac, windows 10 laptop, and I even used the phone’s bluetooth in the companion app but in all cases it seems erratic once I am in game.
After several attempts to delete and recalibrate the hrm, and use different laptops/phones, I’m wondering if its a compatibility issue or if its actually broken?!

Tomorrow I plan to (re)test it with the wahoo phone app to see if the cadence sensor is actually the problem … but if it turns out to be reporting correctly in there, then I am out of ideas what I can do to fix this except buy something else, which is difficult in the current climate.

Any sensible suggestions welcome