File: public_html/app/index.html
URL: …
…
Reaching this at app.alphamethodx.com confirms the subdomain
resolves to its own folder and the apex rewrite did not swallow it.
The negative test is alphamethodx.com/app/, which must return
404. If it serves this page, the same files are reachable from two URLs on
the domain you want indexed.
Try /some/deep/route. The SPA fallback should return this page rather than a 404, which is what React Router needs in production.