The reverse proxies sometimes support TLS pass through (see Traefik). If the reverse proxy puts an authentication page in front, sure, the TLS pass through may not work. But it could work if all you need from Cloudflare is its firewalls, restricting the IP range, hiding your IP, rate limiting, DDoS mitigation, not having to open port in internal servers, etc.
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The reverse proxies sometimes support TLS pass through (see Traefik). If the reverse proxy puts an authentication page in front, sure, the TLS pass through may not work. But it could work if all you need from Cloudflare is its firewalls, restricting the IP range, hiding your IP, rate limiting, DDoS mitigation, not having to open port in internal servers, etc.