Why does my radio button binding not work properly?

I have trouble with the binding of radio buttons in Blazor. I created a blazor fiddle. Please test the following:

1. Open the fiddle.
2. Click some radio buttons and see that the text input is updated properly.
3. Re-run the fiddle.
4. Enter "L" in the input, hit enter and see, that the L-button is selected.
5. Click the U-button.
6. Again, enter "L" in the input, hit enter and see, that the L-button is not selected.

What is wrong here?