Result: built test is not shown in the Test
For me, I changed from using direct DLL references to using the NuGet references of version 3.1.7.4 of SpecFlow and this solved the problem.
I needed to reference "SpecFlow", "SpecFlow.NUnit" and "SpecFlow.Tools.MsBuild.Generation". and then clicked the "Show All Files" button in Visual Studio to see the code behind file at a sibling level to it associated Feature file (this used to be an associated child file of the feature file in older versions).
I also need to use the "Include In Project" context menu item on the code behind file, for tests to show up in Visual Studios "Test Explorer".
NOTE: I did try to unzip the Specflow required files from the NuGet package and reference them directly in my project to be consistent with other direct references in my project (non-NuGet references) but this doesn't seem to work and you have to use the NuGet reference. I found an open issue in github, which states that we have to use the Nuget reference to get the tests to work in Test Explorer: https://github.com/techtalk/SpecFlow/issues/1617