RedundantArgumentMatcherException in azure pipeline when field is used and not when using local variable

Question

I have an issue that only occurs in my Azure pipeline when running unit tests. The unit tests fail when the methods in the test use the field rankGeneratorinstead of a local variable of the same field. The class examples are written below.

This first code block has rankGenerator as a field, and the pipeline fails with the error message: NSubstitute.Exceptions.RedundantArgumentMatcherException : Some argument specifications (e.g. Arg.Is, Arg.Any) were left over after the last call.

However, if rankGenerator is first declared as a local variable, then the pipeline doesn't fail. Does anyone have an explanation for this behavior?

Failing test example:

public class RiskTests
{
    private readonly IRankGenerator rankGenerator = Substitute.For<IRankGenerator>();
    private const string OriginalRisk = "Danger danger";
    private const string OriginalRank = "Rank 1";
    private Risk sut;

    public RiskTests()
    {
        sut = Risk.Create(CreateTestActivity(), OriginalRisk, OriginalRank);
        rankGenerator(
            Arg.Is<string>(x => x == null || !string.IsNullOrEmpty(x)), 
            Arg.Is<string>(x => x == null || !string.IsNullOrEmpty(x)))
           .GenerateRank().ReturnsForAnyArgs(_ => "000001");
    }

    [Fact]
    public void Should_Add_Mitigation()
    {
        sut.AddMitigation("Do this to be safe", rankGenerator);
        
        sut.Mitigations.ShouldNotBeEmpty();
        sut.Mitigations.Count.ShouldBe(1);
        
        sut.AddMitigation("And this is important", rankGenerator);
        sut.Mitigations.Count.ShouldBe(2);
    }
}

Passed test example:

public class RiskTests
{
    private readonly IRankGenerator rankGenerator = Substitute.For<IRankGenerator>();
    private const string OriginalRisk = "Danger danger";
    private const string OriginalRank = "Rank 1";
    private Risk sut;

    public RiskTests()
    {
        sut = Risk.Create(CreateTestActivity(), OriginalRisk, OriginalRank);
        rankGenerator(
            Arg.Is<string>(x => x == null || !string.IsNullOrEmpty(x)), 
            Arg.Is<string>(x => x == null || !string.IsNullOrEmpty(x)))
           .GenerateRank().ReturnsForAnyArgs(_ => "000001");
    }

    [Fact]
    public void Should_Add_Mitigation()
    {
        var localRankGenerator = rankGenerator;
        sut.AddMitigation("Do this to be safe", localRankGenerator);
        
        sut.Mitigations.ShouldNotBeEmpty();
        sut.Mitigations.Count.ShouldBe(1);
        
        sut.AddMitigation("And this is important", localRankGenerator);
        sut.Mitigations.Count.ShouldBe(2);
    }
}

Pipeline definition

   - task: DotNetCoreCLI@2
     inputs:
       command: 'test'
       projects: '**/RigPlan.sln'
       arguments: '--filter CI!=false'
       testRunTitle: 'Run .Net Tests'