Hi,
I haven’t tested for this with ZeroTier, but I know that outside of ZeroTier this type of issue can happen with regular RouterOS 6.x if you use an interface as the “gateway” instead of an IP in the case where an interface must be used. For instance you could use ether1 as a gateway for a connected route only, otherwise if the route is static or something else, an IP must be specified instead of just “ether1” as ethernet interfaces can have many IPs. If you accidentally add a static route that uses “ether1” as the gateway you can end up with the interface trying to do ARPs for internet addresses, and then the ARP table can fill up.
See this thread: http://forum.mikrotik.com/t/arp-table-overflow-please-consider-increasing-max-arp-entri/84261/1