i switched to fire tv to check stations against the roku, when there is an issue with 5.1.
i found that both work correctly, giving dolby 5.1 when available
i ALSO found that disconnecting and reconnecting the hdmi cable had a bearing on whether either would deliver dolby 5.1.
1. the marantz has bug
2. cable quality including 2 way communication ability is very important
3. factors unknown, when it goes out i disconnect hdmi cable restart everything and hope it works again
sorry i cannot offer further remedies.