How to reproduce:
1. Open user's attached project
2. Enter the play mode and observe the console
3. Exit the play mode and observe the console
4. Close Unity Editor
5. Reopen the same project
6. Enter play mode and observe console

Expected result: Console will print out "20" as it was saved to playerPrefs file
Actual result: console prints out "1"

Reproducible with: 2017.2.1f1, 2017.4.9f1, 2018.1.6f1, 2018.2.4f1, 2018.3.0a10

Notes:
- Editor seems to recreate/overwrite playerPrefs file deleting any saved data
- On Linux, PlayerPrefs can be found in ~/.config/unity3d/[CompanyName]/[ProductName]
- Seems to happen only on Linux machines