Dotnet Unit tests failed once but works on the second run

Question

I'm facing a problem when I want to run unit test on my c# project. I have the same behaviour using Visual Studio or command line. Here's my process:

1. Ensure that project is clean (bin and obj folders are not present)
2. Build myproject.sln
3. Run test

1st run : I have failure because path in App.config file is null

3. Run again (without clean, rebuild or something else) 2nd run : all tests passed

I took a look at this topic : Unit test works only on second run but doesn't work for me because I already have the last version of NUnit framework.

I quite beginner on Dotnet so I think I missed some informations.

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EDIT :

This topic app.config not beeing loaded in .Net Core MSTests project solved my problem. It seems that compatibility isn't as obvious as I thought.

Anyway, can someone explains why this problem seems to solve itself when launching 2 run in a row?