Azure Pipeline - launch server inside test application

Question

I have a XUnit project, that when starting up initializes a server in the class constructor:

public OPCUASampleServerTest(OPCTagList oPCTagList)
{
    MySampleServer server = new(true, 3600, null);
    this.thread = new Thread(() => server.Run());
    this.thread.Start();
    this._taglist = oPCTagList.Taglist;
}

The tests utilizing the server, communicates to it via localhost and all works fine on my machine. I have also tested it by cloning the code to assure that everything is present in the repository. Everything compiles as expected. I am trying to have the same result on an Azure pipeline. I have a working pipeline that builds and runs the initial test cases, WITHOUT the server. Now after adding this OPCUASampleServerTest the test fails on the server, but not on my machine. The error is that it is not able to connect to the server

OPCUAClass opc = new("localhost", "51210", this._taglist, 300, false);
bool connected = opc.InitializeOPCUAClient(); //returns false on server, and true on running it on my own machine

I am therefore thinking that the issue is that it cannot see/connect to localhost. Is there some specific setting I am missing in my test function in the pipeline that looks like this:

- task: DotNetCoreCLI@2
  displayName: Test xUnit
  inputs:
    command: test
    projects: '**/OPCDLL.Test.csproj'
    arguments: '--configuration $(buildConfiguration) --collect "Code coverage" --no-build'
    testRunTitle: 'Generic Repository Tests'
    publishTestResults: true

Running what I interpret as the same function using the dotnet CLI:

dotnet test OPCDLL.Test.csproj  --configuration Release --collect "Code coverage" --no-build --framework net5.0

I get 4 passed tests, as it should be. So there is definitely something spurious in the azure run?