Despite successfully building mongoc with OpenSSL 4.0.0 (following CDRIVER-6308), the authentication tests against Atlas fail with the following error:

TLS handshake failed: error:0A000126:SSL routines::unexpected eof while reading calling hello on '(...).mongodb-dev.net:27017'

Current investigation suggests:

• Not an issue that is resolved by setting SSL_OP_IGNORE_UNEXPECTED_EOF (connection close without a close_notify alert).
• Not an issue that is resolved by disabling SSL_OP_ECH_GREASE (GREASED Encrypted ClientHello, aka ECH).
• Not an issue that is resolved by constraining the set of supported groups, signature algorithms, or ciphers for TLSv1.2 and below.