Have not heard of the issue yet, but stick with Support for updates. Was the MX offline for a long time? One issue I've seen before was when a shelf spare MX was actually taken off the shelf and brought online, it wasn't coming up, couldn't establish cloud connectivity, and needed to be removed and unclaimed, then factory reset, so it would first come up in the Meraki admin network, and then re-claim it and add it back into the network. Unless it's something along those lines, and if Support said they're working a known issue, I'd take their direction.
