I bought it from the Microsoft Store, so I haven’t launched the game from Steam.
I was curious, so when I asked “Google teacher” variously, it seems that the 2D screen can be seen in the VR space with the general settings of each game on Steam.
“Use Desktop Game Theater while SteamVR is active”
If this is checked, VR should start in theater mode.
As you say, for the Steam version, unchecking Theater mode seems to be the right thing to do.