Also experiencing this issue. Sometimes redirects work, sometimes they don't. At times I thought I had figured out the logic but there is no 100% consistent pattern, Squarespace's URL mapping is just buggy and annoying.
The most consistent pattern I see is that "https://www.domain" works most of the time, while "https://domain" usually doesn't work. I noticed while setting up an email campaign on Customer.io that Squarespace domains with "www" included are status 200 URLs (successful HTTP requests) while URLs that exclude "www" are treated as 301s (temporary redirect). Screenshot from Customer.io showing this below:
My theory is that by Squarespace isn't good at multi-redirects -> ie the non-www URL has to be mapped initially to the www version, then from there to the new URL, and something along the way here fails and leads to a 404. This is a problem with how Squarespace is handling subdomains and I think there is no solution that customers like us can implement to prevent this (I've tried many and they all fail to patch it). As a result, I'm basically just trying to avoid ever mapping URLs as there are likely a significant percentage of all backlinks that use the non-www version of the link.
Example
Working → https://www.opinionx.co/research-method-guides/thematic-analysis
404 Error → https://opinionx.co/research-method-guides/thematic-analysis
* Not sure if this will work consistently as an example or even if this 100% is the root issue, but this is a frustrating issue that has been affecting my website for a long time now.
@tuanphan