As a followup to 32 bit network prefix via DHCP? it seems that there are two networking stacks running on Meraki routers; One that can correctly use the information provided from DHCP and communicate with hosts on the Internet and one that checks for connectivity back to Meraki Cloud that sends ARP requests out the WAN interface for the host it's trying to communicate with.
This is unusual as devices normally exhibit one of these behaviors and are classified respectively as works with 32-bit prefixes and doesn't work. Which leads me to believe that Meraki routers use at least two networking stacks.
Network traffic looks normal for long enough to think the device might even be working: