You code works where the function the_excerpt is used. It does not work when get_the_excerpt is used, which does not apply the the_excerpt filter. It applies get_the_excerpt instead.
Using get_the_excerpt will cause the filter to apply to both the cases since the_excerpt() uses get_the_excerpt().
Automatically generated excerpt content is a different thing since it is created by a filter on get_the_excerpt that strips the markup.