This seems odd. If I add a second email address to the list of "from" emails at /civicrm/admin/options/from_email_address?reset=1 and set it to default, i expect it to be selected as the first option when using "Send an Email' but it continues to be the second in the list.
If i change the Order of the From emails so my default is first, it continues to show as the second when using "Send an Email' (though perhaps a clear of caches would fix it, but fundamentally something seems wrong here.