See this thread:
I was the dummy. Don’t be like me.
TL;DR: there is almost certainly some app or device that “remembers” your trainer and is attempting to “steal” control in the background.
Fitness apps are “greedy” - they want to run in the background, ostensibly to ensure activities are captured even if they’re not in the foreground.
On your bike computer, find the Flux in “Sensors” - there will be THREE: Power, Speed/Cadence, and “Controllable.” Switch the “Controllable” sensor to “OFF” to ensure the bike computer can’t send commands back to it.
In the Tacx training app, you can manually disconnect from the trainer.