All iPads Stage Manager's "split view": other window always "in background", even when not overlapping

[slplss]

Something is clearly off, because interacting with the dimmed window in what was once "Split View" won't make it "active", only stretching it back and forth. But then, it puts the other window "in background". I am on 12.9" 2021 model, latest public release.

example: the window on the right appears to be darker