How to reproduce:
1. Install Notepad++
2. Create a new Project
3. Go to Edit->Preferences->External tools
4. Select "Notepad++" as the External Script Editor
5. Change External Script Editor Args "$(File) -n $(Line)"
6. Restart Unity Editor

Expected results: Args are saved successfully
Actual results: Args are reset to a default "$(File)" args

Reproducible with: 2019.2.0a12, 2019.2.3f1, 2019.3.0b1, 2020.1.0a1
Not reproducible with: 2017.4.32f1, 2018.4.7f1, 2019.1.14f1, 2019.2.0a11

Additional notes:
1. The Args text field may allow entering only one character, then the text field will be out of focus in Unity 2019.2.0a12 - 2019.2.0a14
2. The issue does not reproduce if VSCode is used
3. On Ubuntu, a "Kate" script editor was used. The args are "-l $(Line) $(File)"