I don’t know if you are aware of this, but there seems to be a conflict between the Lazy Load Media and the new WordPress 6.4 lightbox feature.
Precisely, when the Lazy Load Media setting is switched on, clicking on an image to expand it will not load the image. Instead, only the ALT tag will be shown (if any is set on the image).
I replicated this both on my local and on my SiteGround-hosted sites with different themes. The lightbox features always works perfectly when the Lazy Load Media in the SiteGround Optimizer plugin is turned off.