I just jumped on the trainer and tried a few things using android and my laptop at the same time.
If the trainer is connected to TR via BT then it won’t work with the Tacx app. And vice versa.
One BT connection at a time. Probably a good thing considering all the problems I have with other devices that allow multiple connections.
Do you have multiple apps open at the same time when this happens.
PS. That app only does erg mode as far as I can tell. At least in the power based workout mode I was using in that pic.