I'm having issues with this as well. I recorded what was happening here: https://www.screencast.com/t/oA02fvwajux and https://www.screencast.com/t/yZjJFnfCsl. Tried everything to fix it (cleared my cache, restarted everything, tried it in Chrome and Safari, updated everything, made new pages and started from scratch, etc.). I've reached out to Squarespace and they've said "I can confirm this is related to an issue with gallery sections on our end. We’ve reported this to our Engineering team, and they’re currently prioritizing the issue." They gave me no estimation for fix time. Gonna probably go with the third party app Gallery Block. It's a shame something so fundamental to a website is this buggy.