My site just started being indexed by Google. It was telling me that a subpage was throwing 404s. When I copy/paste the page into the URL bar, it loads fine. The issue was Amplify defaults for "Rewrites and redirects", which didn't work for my React/Router app.
The defaults Amplify gave me were 404's for <> and a regex that redirects non-html pages. I removed the <> and changed the other one to a 200 instead of 404, and now it works correctly. Screenshots of Amplify (broken and fixed, respectively):