I created a fresh Xamarin Forms solution, upgraded all the NuGets, made sure that the UWP version was targeting build 16299, and made sure the .NET Standard project is targeting 2.0. I ran the project and was able to debug the .NET Standard 2.0 DLL fine. You can download this here but I don't think it will be of any use: https://www.dropbox.com/s/jw13heu98yq2n6x/CleanXF.7z?dl=0.
I then pulled these projects in to another solution, added some references to other projects, and copied and pasted some Xamarin Forms pages etc. in to the new clean projects. I could then compile and run the project, but debugging the .NET Standard 2.0 Xamarin Forms project does not work at all. Debugging the UWP code does work, and the lower level .NET Standard 2.0 projects do debug. It's only one project that won't debug. I have already tried Git cleaning the folders again and again to ensure that rebuilds are happening. I have also tried creating the project from scratch and adding all the files back in. Again, it compiles and runs, but doesn't debug.
What things can I try to get debugging working?