I'm having this same issue. Just took delivery and set up the soundbar to my QN85DA TV today. It's connected via eArc. Works fine with Amazon Music and so far it worked fine with Amazon Prime Videos played through that app. But I'm watching a movie through Disney Plus and the soundbar cuts out about every 4 or 5 minutes, flashes eArc and Dolby Atmos lasts 2 seconds and then repeats the same flash. Will work for about 4 or 5 minutes and do it all over again. Both are connected via SamsungThings and as far as I can tell both have updated software. This should not be an issue for what I paid, almost 3k, to have BOTH these products work together since this soundbar was recommended for these types of TVs. If I switch to WiFi connection I get the same issue, but nothing flashes on the soundbar. Would be nice if Samsung stepped in to answer why this is happening.
For the last two weeks, the combo of QN90A and Q800T with Xbox running through TV on eARC as intended has led to all sorts of Dolby Atmos dropping.
About every 10-15 minutes. Was fine from September until now.
Believe it's an update. Notably, the Xbox sound settings have changed, but can't rule out a change Samsung side as updates are automatic.
Predictably nothing from Samsung.
But it did resolve itself with subsequent updates.
I have TV Pass through and Xbox Series X passthrough enabled if that helps.