All of the URL mappings I have configured in squarespace do a 301 redirect from the old slug to the new slug on HTTP. Then the new slug on HTTP corrects to HTTPS via 307 redirect. I haven't found any other forums with the same issue.
Example:
URL Mapping configured:
/insights-innovations -> /blog 301
Resulting redirect chain:
Expectation:
The 301 redirect to http:// shouldn't happen. It should just do a 301 to the new slug on https://