Hi Paul and Delsol,
I'm experiencing a similar problem!
I already had HSTS enabled. But the issue I'm experiencing is when I try to enter my portfolio with my bought domain (via Squarespace, https://www.emtemark.com/) on my computer laptop at work. I get the message "not secure" on chrome and safari. However, I can access the portfolio by going via the longer/original link (https://emtemark.ssquarespace.com/).
But strangely, when I try the same at home with my work computer it works fine to access my portfolio on the bought domain...
Is this a bug since I entered my portfolio on various occasion on my work computer and I just need to clear cache or so? (will need to try it next time in office...)
thanks!