My Samsung TV and Soundbar are 5 years old. I am trying to use the multiroom app to connect my Soundbar and two rear R1 speakers. This problem just started for me a few weeks ago. My system would occassionally drop out in the past but nothing like recently. Now, I cannot keep it connected. It constantly drops the array and reverts to the TV internal speakers. I have tried restoring everything to factory, at least the TV. Not sure I did the sound bar (all I did to the soundbar is unplug and replug). By the way, the TV drops the soundbar even when I am not using the multiroom app, but just the bluetooth Soundconnect for the soundbar only.
This is very frustrating.
I fixed this issue for myself.
I had the exact same issue but with a Pioneer amp connected both by HDMI (arc) and Optical. It kept reverting from Optical to TV speaker on its own just like you explain.
What I did was to switch my amp from the HDMI input to, in my case, "TV input" (now registers as HDMI output on the TV instead of "Optical/Audio output" like before) and the issue went away ... For now at least.
Whether an external soundbar or audio receiver is to be used, the "revert to TV Speaker" scenario is something that happens all the time and is a massive problem for Samsung. I can't for the life of me understand why Samsung cannot enable a software fix to either return back to the days of "disabling" internal speaker or at least remember last setting and "always" turn on the TV with these last settings used. This is of course some kind of "handshake issue" between AnyNet, HDMI, ARC, eARC etc. but too much technology some times ruin the user experience.
This is crazy that it isn't fixed. Can't even blame another manufacturer because it is a Samsung TV and a Samsung sound bar!!! Come on Samsung get this fixed. It's as simple as letting me turn the tv speaker off and letting it stay off.