FYI this update has many improvements (especially better touch gesturers), but has introduced one major issue. With the Duo in Book mode you may encounter a single screen with the other screen black, as if the device was in the Phone mode. A quick rotation action (several may be necessary) turns to black screen back on and the Book mode is correct with two screens on. Also, in YouTube when viewing a video in portrait mode and switching to landscape on the right screen and then using the device in Tent mode (where the back screen turns off) you may experience an upside down screen. Several quick rotation motions usually corrects this.

Clearly this update is the interpreting the accelerometer in strange ways. Hopefully Microsoft will fix this soon.

Added 4/26/2021: An update to the default Microsoft Launcher seems to have mitigated much of this bad behavior.