My brother has 4k tv and we had the same brightness/contrast issue up on fist time connecting xstream issue,
we did factory rest of all three devices issue got resolved.
it my borother setup home theatre (HT) is between xstrema --> HT ---> TV
resetting HT worked .
But in your case xstrema --> TV and soundbar is ARC , so it could be xstrema or TV,
but we are not seeing any issue with xstrema.
By the way, what is systme Diagnostics --> HDMI display in your xsteam ?