TLSProtocol is ignored -> TLSv1.3 is implicit accepted

proftpd 1.3.5e-1build1
on 18.04.4 LTS (Bionic Beaver)

I have problems using TLSv1.3 connections.

Even with the configuration:
TLSProtocol TLSv1.1 TLSv1.2

the logs show TLSv1.3 connections, which should not be allowed:

mod_tls/2.6[10213]: TLS/TLS-C requested, starting TLS handshake
mod_tls/2.6[10213]: client supports secure renegotiations
mod_tls/2.6[10213]: TLSv1.3 connection accepted, using cipher
TLS_AES_256_GCM_SHA384 (256 bits)
mod_tls/2.6[10213]: Protection set to Private

The following negative directive is not known ans leads to startup errors:
TLSProtocol TLSv1.1 TLSv1.2 -TLSv1.3

Already discussed here: http://bugs.proftpd.org/show_bug.cgi?id=4389

Needs patch for ubuntu!