I have also had this issue. Note below on what I did to fix it or what I believe was the cause. Started sometime in Oct.
Trainer: Tacx Vortex
Device with Zwift App: Windows 7 PC, running Companion
ANT + or BLE: ANT to PC
ERG mode or SIM: ERG
I found that I needed to turn off the Companion BLE connection in the App in order to solve the problem. It seemed there was some confusion with the ANT connection with PC and BLE connection to the Companion App. I now just run the Companion App without any control of the devices.