System.Text.Json source generation not working

Question

I'm trying to add System.Text.Json source generation to one of my projects, a .net6 rest api.

When I try to specify the partial class according to instructions I get this behavior:

vs thinks this won't build properly

But I can still build the project, but no files are source generated.

If I just start a new project, using the ASP.NET Core Web API template and add the exact same code it works and inspecting the source generation analyser shows the files being generated. I tried looking through solution and project settings to figure out what could be causing this, but I don't see why this would happen.

The project I'm trying to do this for used to be a .netcore3.1 project, but it has been migrated to .net6, but maybe there's something there that prevents this project from using source generation?

Here's the classes for reference, not that there's much in them:

namespace Test
{
    public class TestClass
    {
        public int MyNumber { get; set; }
        public Guid MyGuid { get; set; }
    }
}

namespace Test
{
    [JsonSerializable(typeof(TestClass))]
    internal partial class TestContext : JsonSerializerContext
    {
    }
}