Build errors when using CommunityToolkit.MVVM with Visual Studio extensions

Describe the bug

I've asked around on SO and discord, but getting no joy. Forgive me if this is my misunderstanding. My SO post: I'm getting build errors despite the SourceGenerator apparently working correctly. Example code:

namespace VisGit.ViewModels
{
    public partial class MainViewModel: ObservableObject
    {     
        [ObservableProperty]
        string _exceptionMessageText = "Exception Text";

        public MainViewModel()
        {
            ExceptionMessageText = "Test Change";
        }
    }
}

Intellisense shows the public property ExceptionMessageText as being generated successfully:

Also, source generator appears to work:

However, I get the build error The name 'ExceptionMessageText' does not exist in the current context:

Full code: https://github.com/stigzler/VisGit/blob/1-setup-base-extension-environment/VisGit/ViewModels/MainViewModel.cs

Not sure what I'm dong wrong. I'm using .net Framework 4.8 (is that the issue?). Any ideas?

Regression

No response

Steps to reproduce

1. Create a new VSIX Project w/Tool Window project
2. Create any Class creating a new ObservableProperty via the Attribute

Expected behavior

priavte var is converted to Public property and usable in same class. Project also builds without error "The name '[x]' does not exist in the current context"

Screenshots

See OP

IDE and version

VS 2022

IDE version

4.8.09032

Nuget packages

[ ] CommunityToolkit.Common
[ ] CommunityToolkit.Diagnostics
[ ] CommunityToolkit.HighPerformance
[X] CommunityToolkit.Mvvm (aka MVVM Toolkit)

Nuget package version(s)

8.2.2

Additional context

No response

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CommunityToolkit / dotnet