2023.01.13 14:03:42 LOG6[16572]: TLS accepted: new session negotiated
2023.01.13 14:03:42 LOG6[16572]: TLSv1.2 ciphersuite: ECDHE-RSA-AES256-GCM-SHA384 (256-bit encryption)
2023.01.13 14:03:42 LOG6[16572]: SSL_read: Socket is closed
2023.01.13 14:03:42 LOG6[16572]: TLS socket closed (SSL_read)

The log says that your stunnel server has successfully negotiated TLS 1.2, and then your TLS client has closed the underlying socket without sending any alert required by RFC 5246.

It's hard to guess *why* your client has closed the socket.  For example, a prematurely terminated (possibly crashed) client could cause such behavior.

Best regards,
    Mike