I reached out directly to squarespace customer service and received a how to fix for this issue that I can confirm works! see below
I had a look at this and I do see what you're referring to. I also noticed something. If you go into Edit and only enable reCAPTCHA, it isn't giving you the ability to save my changes. If you edit anything else in that newsletter block, however, it does.
If you go into Edit for that page, then enable reCAPTCHA, change one other thing (it doesn't matter what) in that block, or even outside of that block so that you get the option to save your changes instead of just clicking Done then it will work in publish.