This is a regression introduced by src/libs/utils/quickwidgetproxy.cpp (added for osgearth).
Making the proxy use a QQuickWindows instead of a QQuickWidget (see quickwidgetproxy constructor) restores the correct full screen behavior.
But this will also cause osgearth views to behave badly when being resized rapidly.
This issue is hard to fix.
I suggest to keep it as a "known issue" in the next release.