Client certificate authentication fails with iOS 13.4.1 (works with 13.2.2)
Requirements for trusted certificates in iOS 13 and macOS 10.15

Does the client certificate meet Apple's latest certificate requirements?


  All TLS server certificates must comply with these new security requirements in iOS 13 and macOS 10.15:
  
  
  TLS server certificates and issuing CAs using RSA keys must use key sizes greater than or equal to 2048 bits. Certificates using RSA key sizes smaller than 2048 bits are no longer trusted for TLS.
  TLS server certificates and issuing CAs must use a hash algorithm from the SHA-2 family in the signature algorithm. SHA-1 signed certificates are no longer trusted for TLS.
  TLS server certificates must present the DNS name of the server in the Subject Alternative Name extension of the certificate. DNS names in the CommonName of a certificate are no longer trusted.
  


While the document mentions server certificates, I suspect client certificates will also be rejected if they using SHA-1 or weak keys.

Debugging with openssl s_server

Try debugging the TLS connection with OpenSSL's test server, see Verify Incoming SSL Using OpenSSL S_Server for a related question.