The “default” value is not selected as default in the form even this value is “Yes”
is it a bug?
It depends. If you have your form in standard mode it will show you the situation of the record in the database. If you hit ‘new’ then the value should appear as the default value. That works well as we use it all the time. So if it doesn’t other things might apply like setting the variable in events, using ‘use title’ etc.