Hi Slate,
This is a little older post but I was having this exact same issue when connecting a WAN 2.
after talking with Meraki support they had me upgrade to the latest beta software but that still didn't help.
It turned out that it was the module.
In our WAN 1 we were using an RJ45 to sfp MA-SFP-1GB-TX that was working but in out WAN 2 we were using an GLC-T module which had same behaviour as you are describing.
Cisco should definitely have some documentation on which ones you can and cannot use. huge waste of time.