To recreate:
Open Projucer on windows 10.
Resize window by dragging edges.
Press full-screen on title bar.
Close Projucer from title bar.
Open Projucer again and the window is not displayed full-screen (although the full-screen icon on the title bar shows the windowed/two-box icon).
Same behaviour when running the extras/Projucer
project with the latest tip of the develop branch.