Yo I have been dealing with a similar issue and this ended up solving my problem: https://medium.com/@monarchwadia/github-pages-setting-up-www-subdomain-with-ssl-https-aca9eca371d6 Maybe try this out and see if it does anything for you? Basically in the Github pages repo settings, I just had to make sure "www." was in my custom domain and "enforce https" was checked. For some reason it wasn't working when I just had the apex domain entered as the custom domain in settings. Our problems are different since I was having the opposite problem, where the apex domain was working but it just wouldn't work solely when using SSL *and* the www subdomain. Worth looking at though! Hope you manage to fix your issue as well!
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