This post is to inform others of the solution I came across for my long-standing issue between my Xbox One X and my HW-Q90R, where the rear speakers do not play any audio from my Xbox One X after not being utilized in a little while. The rear speakers would both be powered and connected with that solid blue LED indicator, despite the issue, and the recent firmware update having been done for the soundbar and all its peripherals. The problem lies within the Dolby Atmos audio settings for the Xbox One X, where, if you HAVE set up Dobly Atmos for your Xbox, the rear speakers won't know to output any audio. One must go into the Xbox's General settings, then Audio, and change from Bitstream (for Dolby Atmos) to 7.1 uncompressed audio (or anything other than Bistream) in the middle column. The rest of the audio settings seem to have no effect on the rear speaker output, except for that one. Once you switch to anything other than Bitstream audio, the rear speakers should begin to work normally immediately. I know that this post is extremely niche, however, if you have Dobly Atmos set-up for your Xbox One X to be utillized with your HW-Q90R, you might be missing out on your surround sound entirely. I hope this helps someone else, somehow took me 2 months to figure this one out...
I had the except same problem! The problem with the xbox one is the way it processes atmos and non atmos content. If for example you watch a netflix show which has true dolby atmos channels they will actually work (games like assasin creed origens will work too perfectly with rear speakers giving full volume) .. if the show is 7.1 or 5.1 the xbox will try to simulate 5.1 or 7.1 in dolby atmos channels (it never turns dolby atmos off). Somehow it will mess it up completly (not only with samsung bars with any atmos reveiver tried allot unless you use multiple speakers for dolby atmos but it still messes where the sound should be) and no rear sound will be there or really soft. So you have to indeed keep switching between atmos when watching or playing atmos games over hdmi and for me bitstream dolby digital will work fine too for 5.1 content or 7.1 content. Microsoft support will confirm this but because its part of dolby acces software which you have to install they keep saying its not there problem and dolby keeps saying its microsoft (i have send multiple emails and gave up).
So until dolby will fix channel upmixer in the app we have to keep switching within setting of the xbox itself. Dont forget if you put it on 7.1 you have no dolby atmos as the up speakers will be disabled.