Not Converting PNGs to JPEGs on Custom Theme

"I'm using the fantastic WP Smush plugin, I've upgraded to Pro, I've enabled the "Convert PNG to JPG" setting, but running a smush doesn't return JPGs. Any ideas? When I check in the media library, I still see file type: PNG.

These are not transparent PNGs and converting them to a JPG would definitely have some file size savings."

Member is using a custom built theme, and not all of their images are being converted over from PNG to JPEG.

Member is referencing the image src url with this function:

<?php the_post_thumbnail_url('video'); ?>

video is a custom size added to his functions.php.

Need some added support on this one team