Works perfectly fine for me.
I have a empty function.php in my child’s theme, however, it has that one line:
<?php remove_action( ‘init’, ‘bp_dtheme_add_custom_header_support’ ); ?>
Make sure there isn’t any other function that may override the bp-defaults.
If that doesn’t work, you can attempt to copy the function and replacing the the few lines with a return call.