Thanks for taking the time to post!
There could be a few things at play here, so I’ll try to address them for you succinctly.
It could be that the communication between the game and your trainer is causing issues with the way the cadence is being transmitted, so I’d like to ask if you can check your signal strength for your trainer connection. There is an article on our support site that will help you with specifics if you need.
You can also try changing your connection type. If you’re using ANT+, you can try Bluetooth via your machine’s native Bluetooth or via Zwift Companion. If you’re on Bluetooth, ANT+ might help as well.
If that doesn’t help, you can try calibrating your trainer on the Tacx Utility app if you’ve not already done so. Instructions for that are found on their website.
If those steps don’t help, I would like to ask you to write into us at firstname.lastname@example.org and send us your game log files so that we can take a deeper look for you. Instructions on finding those files are located here on our support site. You can ask for me, Flint, or any other of our other agents would be happy and capable to help you as well.