RelayCommand and refresh content not work

[yamamotokazue]

I'm WPF beginner, I've encountered a problem.
background:
IDE:VS2022
.net Framework 4.8.1
CommunityToolkit.Mvvm 8.2.2

here is my code.
ViewModel

public partial class MainWindowViewModel : ObservableObject
{
    [ObservableProperty]
    [NotifyPropertyChangedFor(nameof(TextUpper))]
    private string t1;

    public string TextUpper => t1?.ToUpper();

    [RelayCommand]
    private void SyncMethod() 
    {
        t1 = "click";
    }
}

Xaml

<StackPanel>
    <TextBox Text="{Binding T1}" Background="Blue"/>
    <TextBox Background="Red"/>
    <TextBlock Text="{Binding TextUpper}"/>
    <Button Command="{Binding SyncMethodCommand}" Content="click"/>
</StackPanel>

the question is when I modify blue textbox, textblock will refresh content, that's good!
but when I click the button, do SyncMethodCommand, and t1 value changed to 'click' string,
but blue textbox and textblock didn't replace to 'click' string on view.
Did I miss something?

Thank you for your response.

===update.
in .net core 8, everything are fine.

    internal partial class MainWindowViewModel : ObservableObject
    {
        [ObservableProperty]
        [NotifyPropertyChangedFor(nameof(TextUpper))]
        private string t1;

        public string TextUpper => t1?.ToUpper();

        [RelayCommand]
        private void SyncMethod()
        {
            T1 = "click"; //<--- t1 to T1
        }
    }

so I back to 4.8.1 and t1 changed to T1,
but got some error (error CS0103).
can it be used in .net framework 4.8.1?