I've been having the same problem as others on here with Cloudflare, i turned proxying off and fiddled with some settings until it changed from "cloudflare detected" to "issuing TLS certificate" but now it doesn't complete issuing and the site is still using the railway certificate causing the browser security error:

the site works here: plausible-hour-production.up.railway.app

but not on my custom domain: ftxvote.com

Ive busted the cloudflare cache, set ssl to full, turned off dnssec, turned of proxying, turned of hsts and tls 1.3, "always use https" is on, the dns has the CNAME at the root