To reproduce:
1. Open attached project "1111198.zip"
2. Select "GameObject" from the hierarchy
3. Observe in the Inspector window that there is a gap between "Testing" and "Age" variable
4. In Player Settings change Scripting Runtime Version from .NET3.5 to .NET4.x
5. Select the "GameObject" again and observe as there is no gap between "Testing" and "Age" variable but there is a gap between Script name and "Testing"

Expected result: [Space] attribute works on .NET4.x in the same way as in .NET3.5
Actual result: [Space] attribute is placed before [Header] attribute even though in the script the [Space] attribute is after [Header] attribute

Reproduced on:
2017.4.0f1 2017.4.22f1 2018.3.8f1 2019.1.0b6 2019.2.0a7

Notes:
If [Space] attribute is added before [Header] attribute, then space behaves normally.