Thanks a lot!
That fixed it and stopped us from worrying something is wrong.
We are actually using this setup with a transit network to a SD WAN setup and were also pinging through that. This means not through the default LAN.
I also used this sk42733 about "Connection from one side of the ClusterXL destined to the physical IP address of a non-Active cluster member on the other side of the ClusterXL fails"
with fwha_forw_packet_to_not_active and with fw_allow_simultaneous_ping active it now works perfectly and monitoring is happy 🙂
(and yes: R77.20 is old but that's the issue with SMB devices, they lag in features and version compared to big Gaia. I wish CP would use one OS everywhere just like most competitors do...)
I hope my reply helps others too.