A:AnswerThat issue will be resolved later this year. It affects any AVR (not just Denon or Marantz) that uses the same HDMI chipset. In the meantime, you can output 4K/60 from the Xbox with no issue or you can connect the Xbox directly to the TV and use ARC to feed the audio info back to the AVR.
A:AnswerNot at this time, but we have a solution for that coming very soon. Currently, you can either connect the Xbox X to the TV directly and then use ARC to feed the audio back to the AVR or decrease the Xbox X's video resolution to 4K/60 and the Xbox X should pass through the AVR.