Thanks—
It doesn't appear to be an issue with my browser caching. Even after I clear my cache, I'm still able to access the AMP version of pages. Are you saying that it will take some time for Squarespace to update their cache?? It's been 2 weeks already so...
I also see that squarespace redirects don't support URLs with parameters (such as ?format=amp). Any suggestions on how to handle the redirect once the AMP versions are eventually taken down?