How to reproduce:
1. Open the attached project
2. Open DoubleDisplay.cs script
3. For methods Display.displays[0].SetParams and Display.displays[1].Activate, change window size parameters to 480x480
4. Build and Run the project

Expected result: When the game is launched, in both displays window sizes are like the ones that are set in the DoubleDisplay.cs script.
Actual result: The primary display window size turns to native. (Second screen has expected behavior.)

Reproducible with - 2017.1.0p5, 2017.1.4p1, 2017.2.0b7, 2017.2.3p1, 2017.3.0a4, 2017.4.5f1, 2018.1.5f1, 2018.2.0b8, 2018.3.0a2
Not reproduced with - 2017.1.0p4, 2017.2.0b6, 2017.3.0a3

Notes:
- After regression, when using only one display, the window size is the same as Screen Resolution that is selected when launching the build (instead of the one that is set by Display.displays[0].SetParams in the script).