Why is this an issue?

A method is identified as a test method if it is marked with one of the following attributes:

However, non-public methods are not considered test methods and will not be executed, regardless of whether they have a test attribute. Additionally, methods with the async void modifier or methods that contain generics <T> anywhere in their signatures are also excluded from being recognized as tests and will not be executed.

[TestMethod]
void TestNullArg()  // Noncompliant, method is not public
{  /* ... */  }

[TestMethod]
public async void MyIgnoredTestMethod()  // Noncompliant, this is an 'async void' method
{ /* ... */ }

[TestMethod]
public void MyIgnoredGenericTestMethod<T>(T foo)  // Noncompliant, method has generics in its signature
{ /* ... */ }

[TestMethod]
public void TestNullArg()
{  /* ... */  }

[TestMethod]
public async Task MyIgnoredTestMethod()
{ /* ... */ }

[TestMethod]
public void MyIgnoredGenericTestMethod(int foo)
{ /* ... */ }

Exceptions

For xUnit, accessibility is disregarded when it comes to [Fact] test methods, as they do not necessarily need to be declared as public.

In xUnit, [Theory] test methods, as well as [TestCase] and [TestCaseSource] test methods in NUnit, have the flexibility to be generic, allowing for a wider range of test scenarios.

Resources

Documentation