On the Garmin Edge, go to settings, sensors, find the trainer. There will likely be THREE entries for it: one as speed/cadence, one as power, another as “Controllable”. The latter is what’s causing your issues.
Either switch that sensor OFF or entirely REMOVE it as a Controllable Trainer.
Photos below from my Edge bike computer connected to a NEO2T, to illustrate the point. 
Speed/Cadence, and 
Power in the first image, Smart (controllable) Trainer in the second. Final is sensor details of the Smart Trainer.
See also this thread: Pssst: Yes, you do have another device connected, dummy
I was the dummy. Don’t be me.
TL;DR: any device that has ever been connected to your trainer is a suspect for taking control of it - many fitness apps are “greedy” and operate in the background, trying to maintain connections. Ensure that every other device - phone, tablet, watch - “forgets” your trainer. That includes configuration/settings apps like those from Tacx/Garmin, Wahoo, Elite, Saris etc.