What to run
Open SSL Expiry Checker. The result is from Cloudflare. It reads the peer certificate. It does not grade the whole TLS config.
How to read the result
Issuer is the signing certificate's subject, not the site brand.
A different issuer after a proxy change is normal.
Expiry still belongs to this leaf.
What to open next
Open HTTP Headers for strict-transport-security if the question is whether you are forcing HTTPS, not who signed. That result is from Cloudflare.