The topic-tags.php template is being explicitly included through the topic_tags() method of template-functions(), so replacing it in my-templates has no effect.
This makes me wonder if, generally, a bb_include and bb_require method could be used for all includes which would check the template system before resorting to a default folder.
this could possibly replace the oft repeated:
if (file_exists( BBPATH . 'my-templates/whatever.php' ))
require( BBPATH . 'my-templates/whatever.php' );
else require( BBPATH . 'bb-templates/whatever.php' );
I thought it worth mentioning because theme developers might find it useful from the outset.