How does zone 2 on your AV receiver work if you connect a source using HDMI directly to the receiver? Like, if you hook up a game console or a laptop with HDMI out? Do you get audio in both your main zone and zone 2? If so, then I'd switch over to a stand alone Roku Ultra and go with that and simply bypass the internal source of the TV.
At the end of the day, the Roku TVs on the market tend to be so-so in quality, at best. These aren't exactly the highest quality in brands. That's fine, but you are now finding out that things can't all work perfectly across all connected devices. What you want to do is well beyond 'basic' setup, so you will need to change things up a bit I expect.