I set up the site on GitHub Pages, but HTTPS would not provision. Namecheap still had a URL redirect from @ to www.usemosey.app. It looked harmless, but it made the root domain resolve through an extra IP.
Removing the redirect, confirming that only GitHubβs four A records remained, and re-adding the custom domain in GitHub Pages fixed it.
Future reminder: URL redirects can silently interfere with GitHub Pages HTTPS, even when the visible DNS records look correct.