Nlbwmon does not register voip traffic

In December 2021 I started the following topic:

https://forum.openwrt.org/t/nlbwmon-catching-voip-traffic/115410

At that time I had activated sdp-forwarding, so the RTP data flows from my IP-phone to the VoIP provider and vice-a-versa. Nlbwmon registered this traffic as "other traffic".

Now I've changed this configuration and deactivated sdp-forwarding. The following picture shows the VoIP data flow:

192.168.52.19 is the WAN IP of my OpenWRT router (22.03.02), 195.185.37.60 the IP of my VoIP provider.

nlbwmon gives the following statistics (172.19.50.1=router, 172.19.50.40=IP-phone):

  Fam            Host (    MAC )      Layer7      Conn.   > Downld. ( > Pkts. )      Upload (   Pkts. )
IPv4    172.19.10.130 (bd:e7:10)    HTTP-TCP      2810    6383985 B (   10385 )   2912742 B (   11913 )
IPv4      172.19.50.1 (c6:0a:50)   YSIP-5060      1674    5497778 B (   12005 )   8223071 B (   68921 )
IPv4    172.19.10.130 (bd:e7:10)   HTTPS-TCP      2608    2331470 B (    1542 )    442423 B (    4993 )
IPv4     172.19.50.40 (dd:cb:94)    HTTP-TCP       486     977242 B (    2409 )    235469 B (    2794 )
IPv4    172.19.10.130 (bd:e7:10)     DNS-UDP        34       1064 B (       8 )       576 B (       8 )
IPv4    172.19.10.130 (bd:e7:10)     NTP-UDP         7        152 B (       2 )       152 B (       2 )

Before the 5-minutes call the YSIP-5060 traffic was:

IPv4      172.19.50.1 (c6:0a:50)   YSIP-5060      1666    5452705 B (   11909 )   8158243 B (   68403 )

The difference is 5497778 - 5452705 = 45073 Bytes. So this difference definitely contains no RTP data. I have expected the RTP data would show up as "other traffic". But there is no "other traffic".

Here is the output of grep -vE 'port=(53|22|123|8853)' /proc/net/nf_conntrack:

ipv4     2 udp      17 178 src=172.19.50.40 dst=172.19.50.1 sport=4060 dport=5060 packets=0 bytes=0 src=172.19.50.1 dst=172.19.50.40 sport=5060 dport=4060 packets=0 bytes=0 [ASSURED] mark=0 zone=0 use=2
ipv4     2 udp      17 298 src=172.19.50.1 dst=195.185.37.60 sport=5060 dport=5060 packets=9 bytes=270 src=195.185.37.60 dst=192.168.52.19 sport=5060 dport=5060 packets=0 bytes=0 [ASSURED] mark=0 zone=0 use=2

Can anybody explain this behaviour?

Nobody any ideas why nlbwmon does not correctly register VoIP traffic?