TLDR: Update the testing packages, look into the output -> test console
I struggled with this for a day and a half. so here's what I did to solve it:
Since all the packages were updated during the .net framework update, I started with the differences between the working and not working projects. The first clue was that all 3 projects were using: MSTest.TestAdapter and MSTest.TestFramework
Naturally I went to the -> Output console -> Test dropdown in VS 2019 and looked at the output. Nothing useful there.
Step one of the solution: Update the MSTest.TestAdapter and MSTest.TestFramework to version 2.0.0
Step two of the solution: Now the Output console -> Test dropdown output started showing one after the other, missing packages and wrong binding redirects
Step three of the solution: Manually add the missing packages. For me those were
Step 4 of the solution: Removed/Fixed the unnecessary binding redirects.
I hope this will help someone else.