I turned the TV on using the Xbox activity, and it didn't turn the soundbar on. Well I went into settings on the Xbox, before turning the soundbar on, and Dolby Atmos was selected.

So something to do with the TV is why Dolby Atmos isn't always supported. I restarted the Xbox once, and that didn't fix it. So probably the TV. I could email Vizio and ask for a fix, but I'm lazy.

There's another solution, buy a HDMI switch, and plug the Xbox directly into the soundbar. I think the Apple TV always has Dolby Atmos. Problem is finding the right HDMI switch, might need to be HDMI 2.1. There are some cheap ones, but they don't have that many ports.

Does the Apple TV always have Dolby Atmos when using eARC from the TV? If so, I could just plug the Apple TV directly into the TV.

If I get a PS5 someday, I'll need a HDMI switch though. Most of them only have two ports, so looks like the Apple TV gets to stay plugged into the soundbar.