I have just had a reply from a team leader at Sqaurespace about my issue with the gallery, which I have copied below.
I find it unbelievable that they can make such a fundamental change without either asking, or informing, its thousands of users, and all because someone asked for it in a forum.
I moved away from wordpress because I wanted a more stable platform, where I didn't have to worry about updates changing things any more. I spent a lot of time testing my site to ensure it worked perfectly before the changeover, and yet less than two months later, I am in a worse position than when I started.
I will now have no other option but to find an alternative to squarespace, as I can't be sure they won't change another fundamental component of my site without my consent or at the very least informing me beforehand. Yes, I tried the CSS and it split an image with a text block, so it basically didn't work. Even if it had, who is to say they won't change something else that I can't fix with CSS.
Can anyone suggest another provider, similar to squarespace but lets you decide what your site will look like.
My name is NJ and your ticket has been escalated to me for further review. I understand you have reached out via Twitter as well, and I appreciate your patience as I worked to get to your case.
I see that the mobile display of your homepage has changed recently from a single column, to multiple columns. Thank you for sending along the screenshot and cached link to your site. When I pull it up on my iPhone XR, I'm seeing the same behavior - the single column on the cached site from June 7, but the 2 columns as confirmed yesterday on June 9.
I can certainly appreciate your position when the site changes in a way you didn't anticipate or hope for. We strive to keep these types of adjustments as infrequent and calculated as possible when/if they will impact active sites. Like any technology, future updates are a reality, but you shouldn't expect visual changes of this nature at a high frequency.
From my research, it appears that this update was intended to provide a more consistent appearance between the mobile and desktop versions of sites. In fact, the article linked below is from our forum, where a user asked about creating this behavior with CSS:
In order to adjust your site's mobile appearance, back to a single-column, you'll need to use CSS. Although we do not provide or troubleshoot this type of advanced customization, you may be able to adjust the CSS linked above, to create a single column on mobile devices. For additional assistance, I recommend exploring our guide on adding CSS:
Using the CSS Editor
Again, I cannot provide or troubleshoot this CSS, so if you'd like more specific assistance, I recommend replying to the article.
Lastly, I want to acknowledge your request to revert this change for your individual site. We do not offer this type of personalized adjustment and control on a per-site basis. I have passed along your feedback and preference for a single-column on mobile, but I cannot guarantee that this will be available on the Masonry gallery section again, without using custom CSS.
If you have any questions or concerns, I'll be keeping an eye out for your response. As a Team Lead, I'm not as readily available to respond, but I will do my best to get back to you on weekdays.