Hi, I have the sam problem with QE43Q65T QLED TV but I think I found something interesting that could help someone.
I bought TV few days ago. I logged in TV with my Samsung account and SmartThings app on TV showed my 5 scenes and all devices, and everything was working fine until few days when i noticed message "No registered scenes and devices". SmartThings was (and is) still working fine on all my other devices (Garmin watch, Android auto, phones, tablets, Alexa... ) and my new TV is still controllable from all of them, but I lost possibility to control devices and scenes from SmartThings app on TV. While i was investigating what happened I've noticed that voice commands ("run scene") were still working. Bixby offered me my scenes, although they were not available in SmartThings app, and I could choose which one to run, all except the last one that I made the day before. Then i tried to erase that last scene that i made, that was missing in voice activation, and after that all scenes and devices came back. I was playing a little after that with making and erasing scenes in SmartThings app on mobile phone and noticed that after every new scene that i make i lose all my Scenes and devices (old and new) from SmartThings app on TV, even when i was making again old scene after deleting it, so I ended up with just 2 scenes and without possibility of making new one without losing everything from SmartThings app on TV. I've tried disconnecting TV from mains plug for cca one hour, as instructed by Samsung phone support, reinstalling SmartThings app on TV, and even resetting Smart Hub, but nothing helped.