The project window is not maximized, that is true. When it is maximized, the project window's frame disappears and docks with the parent (outermost) window; hence, it is "hidden". However, I am almost (90%) certain that in the past, the project window inherited the outer Windows 10 theme characteristics.
I typically have multiple projects open at the same time, so having multiple windows cascading is beneficial to my workflow.
In the end, this is only a cosmetic idiosyncrasy. Just curious if someone would have a quick fix.