I made some more tests:
I thought, this is surely NOT any CMK issue, because the ping4 and ping6 from inside the cmk container are failing occasionally, too!
So I opened up another docker container and I dont see any issue inside it?
I started two parallel pings:
The fails are always inside the cmk container! Both containers are configured the same way. Both has the same macvtap interface and both have a static IPv4.
It does not happen with IPv4 in my tests. IPv6 issue?