sock.get_client_ca_list() returns empty result with TLS1.3

[feenes]

I try to get the list of client CAs, that a server allows.

with openssl I can call:
openssl s_client -connect hostname:443 -servername hostname and I can see the CAs after the lines

---
Acceptable client certificate CA names

for most servers I succeed with following code, that I borrowed from stackoverflow

import socket
from OpenSSL import SSL

def get_client_cert_cas(hostname, port):
    ctx = SSL.Context(SSL.SSLv23_METHOD)
    # uncommenting next line will make things work
    # ctx.set_options(SSL.OP_NO_TLSv1_3)
    sock = SSL.Connection(ctx, socket.socket(socket.AF_INET, socket.SOCK_STREAM))
    # next line for SNI
    sock.set_tlsext_host_name(hostname.encode("utf-8"))
    sock.connect((hostname, port))
    sock.do_handshake()  # without this command next line always returns empty list
    return sock.get_client_ca_list())

For some servers I always get an empty answer. Though the openssl command returns the correct information.
As soon as I disable TLSv1_3 I am able to receive the client_ca_list.

Is this to be expected or is this a bug?