Fortiproxy Cluster loses Sync approx. every 10 minutes and syncs again

I am not sure if this is normal behaviour or a problem?
Our Fortiproxy cluster loses the sync approx. every 10 minutes, then it is out of sync for 1-2 minutes, then it syncs again. I can see it in GUI, on the CLI and also with SNMP.
It is active-passive HA with unicast heartbeat on explicit HA-interfaces in VLAN which is only used for this.
I've already rebootet both devices (VM64 v7.4.2 build0577).
Switchover worked fine.
It is not a network issue!
When I captured the traffic I've not only seen UDP-traffic between the peers, but also TCP on ports 703 and 700.
703 is mentioned in the Fortiproxy port table, but 700 is not in the list (seems to be harelay).
https://docs.fortinet.com/document/fortiproxy/7.4.0/fortiproxy-ports/758533/incoming-ports
Any tipps?
Thanks.