This is what worked for me. Put the entry.Text inside the Dispatcher.
https://github.com/dotnet/maui/issues/25728

Dispatcher.DispatchAsync(() =>
 {
     if (sender is Entry entry)
     {
         entry.Text = texto.Insert(texto.Length - 2, ",");
         entry.CursorPosition = entry.Text.Length;
     }
 });