Steps to reproduce:

1) Download attached project "New Unity Project.zip" and open in Unity.
2) Build project for Windows standalone x86_64.
3) Launch built project in fullscreen mode(with highest resolution).
4) Press F11 to exit fullscreen mode, click on Maximize button(R.top).
5) Now Click again F11 to enter fullscreen, and you will notice it enters to fullcreen mode, but resolution which you applied before is not correct.(see attached gif "fullscreen.gif")

Reproduced with : 5.4.0b22, 5.4.0b23.

Not reproduced with: 5.2.4f1, 5.3.5p5, 5.4.0b21.