I have the exact same issue and I reported it before. And it’s occurred ever since Zerotier was added as a package.
If you leave it without restarting the zerotier instance it will eventually come back which makes me believe it’s related to connection tracking in the firewall.
Although I haven’t bothered to check into it in more details as I just restart the zerotier instance.
My previous report: http://forum.mikrotik.com/t/zerotier-added-to-routeros-v7-1rc2/151475/246