Same issue! Unfortunately I did not realize that editing the mobile site would allow changes to flow back to the desktop version, and realized only after hours of minor adjustments that it royally screwed up my entire site. 🤦♀️ I assumed it was a glitch, but to see that others have been having this same issue for more than a year, and Squarespace's only solution seems to be "use a mysterious line of custom code that we have not provided" ... pretty sure I'm done wasting time with Squarespace. There are better solutions out there.