I recently noticed that not all images on my site are showing with the <picture> tag around them.
The images from my theme GP Premium work fine, but the images from WPSP do not have the <picture> tag around them.
I use WP Rocket and Imagify. The Imagify option to display the <picture> is used.
That makes sense, as our resizer kicks into gear when you specify custom image sizes in the list. That image resizer typically won’t work with plugins like filter the output of normal featured images, which are output when no custom sizes are added.
This is a tricky issue as of right now, as there’s no way to target individual lists with that filter.