Reproduction steps:
1. Open Unity
2. Open File -> Build Settings.. -> Player Settings..
3. Set Display Resolution Dialog to Hidden By Default
4. Check or uncheck the Default Is Full Screen

Expected: If Default Is Full Screen is checked the game starts fullscreen if the property is not checked the game should start not full screen
Actual: If Default Is Full Screen is checked the game starts windowed

Reproduced on: 5.5.6f1; 5.6.5f1; 2017.1.3f1; 2017.2.1p1; 2017.3.0p1

Note:
2018.1.0b1 does not have property Default Is Full Screen.
The issue is reproducible on Mac.