My Samsung 2018 Q9F was doing this also with a Yamaha soundbar connected via HDMI (ARC) After 3 weeks of troubleshooting, it came down to the HDMI cable. Even though the cables I tried are printed high speed on the cable, it still didn't work. I tried 3 different cables. What worked was a cable labled that it was arc compatible on the package. Changed the cable, reset the the tv's Smart Hub by using the TV's menu. Turned on the TV and then plugged the soundbar into the HDMI ARC input on the Smart Hub, tv recognized it, changed the sound to HDMI receiver and now everything works. No turning on by itself anymore. Resetting the Smart Hub required me to re-connect to my Samsung account and re-do all of my Apps.
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