I am 100% in the same boat, same situation with no resolve and have spent hours with tech support to be redirected elsewhere. I have spent many hundreds of dollars on UHD movies that are unwatchable during "peak times" and have a gigabit internet connection and am hardwired. 4k HDR content from other services on the SAME device works perfectly as my bandwidth is above average. This is exclusive to Microsoft Store purchased content. I had a tech tell me that since I have a Dolby Atmos Audio system that the problem was Dolby's and needed to contact them for support. They obviously didn't listen to my troubleshooting attempts or my actual issue and have no idea how the technology even works. Felt like they were just redirecting the blame and blowing me off. Horrible experience. Considering legal action to get refunded.
Even after logging more than 1 support ticket for this haven't received any follow up at all and no emails...nothing. Unreal!! Way to erode consumer confidence after I just switched back from going exclusively Playstation when the Xbox 1 X came out and have spent a small fortune on content to take advantage of the console only to be completely ignored as a customer. This issue has persisted off and on for over 6 months. This deserves a class action suit and a whole lot of letters to the Better Business Bureau and the Department of Consumer Affairs. I have the time and the energy!