C# Running all unit tests does not finish when first run

I created new unit tests in a new class. When I go to Test > Run > All Tests, it gets stuck in the opening brace of the first unit test method of that new class. Test Explorer shows the unit test with a "Play" icon, listed under "Not Run Tests". It doesn't seem to ever finish. I click "Cancel", then "Run Not Run Tests", and then it succeeds (finishes all 700+ tests). It also succeeds when I manually run the new unit tests by right-clicking them then "Run Selected Tests"

What might be causing this and where should I start looking?