with Optimus, mobile lazy needs to be disabled (iOS bugs) ⇒ https://docs.wp-rocket.me/article/139-disable-lazyload-on-mobile
Alternative use Imagify...
From: WP Rocket
In this scenario you are using server-side caching from WP Engine and using WebP on your site. When using a different page caching method other than WP Rocket, it is only possible to use WebP by using the Picture Element method from Imagify. Using webp (Optimus) with WP-Engine won't work correctly.