The root cause of this issue is that the iOS keychain imported only the client public and private keys, but not the CA certificate. A PKCS#12 file (.p12 or .pfx) is a file format that contains a certificate and its private key, optionally protected by a password. A PKCS#12 file can also contain intermediate certificates or root certificates that are needed to verify the certificate chain. However, when importing a PKCS#12 file into the iOS keychain, only the certificate and its private key are imported, not the CA certificate. This means that the iOS device cannot verify the authenticity of the certificate, and displays the error message "mbedTLS: ca certificate undefined". To fix this issue, the CA certificate needs to be imported separately into the iOS keychain, either manually or using a configuration profile. Verified Reference:
https://developer.apple.com/documentation/devicemanagement/certificatepkcs12
https://support.apple.com/guide/deployment/distribute-certificates-depcdc9a6a3f/web
https://openvpn.net/faq/how-do-i-use-a-client-certificate-and-private-key-from-the-ios-keychain/