Macvlan driver is from docker as far as I know.I couldn't say anything since this is too bizarre for my limited knowledge..
But glad it helped. Although I also noticed that you would need to re-ping your desktop after sometime to keep the GUI working
Nice security feature huh? lol
You are right, the only reason it would be portainer is if it didn't create the macvlan settings correctly.
But as you also tried it in docker directly with the same result it won't be portainers fault.
Anyway I'm happy you found a temporary solution as this was driving me nuts.
I recreated my macvlan network in every possible way, also tried to ping the container just not the other way around
So I was just ready to reinstall omv, as I thought maybe first have a look in the forums....