I know this is an unsatisfactory answer, but I've started direct-linking to images instead of lightboxing. That is, I add a direct link to each image so that it loads the full-size image naked in the browser when you click on it in a gallery. It's totally hacky and ugly, but if you're trying to proof something for a client on their phone and want them to be able to zoom in on the full-res image ... it's the only free solution I've found.
https://support.squarespace.com/hc/en-us/articles/206542427-Image-clickthrough-URLs#toc-gallery-blocks
I suggest sending a note to the engineering and design team via support and let them know this is important (and kinda ridiculous.)