The <!-- more --> tag isn’t applied via the the_content filter, it’s included in the get_the_content() function, which checks for the existence of the tag within the content using a regex and then outputs the more link appropriately. this output is filtered via the the_content_more_link filter, but since you’re not using get_the_content() you won’t be able to leverage that. you’ll need to perform a similar regex replacement in our custom code. I would recommend filtering your more link via the_content_more_link as well, for compatibility. You can see the code get_the_content() uses here: https://core.trac.wordpress.org/browser/tags/4.0.1/src/wp-includes/post-template.php#L255
