Meraki MX does NOT support LACP for WAN or LAN ports. That means on the Catalyst side you must not use channel-group on the MX-facing ports.
If the Catalyst uplinks are in a port-channel, a failover will break traffic until the LACP state machine times out.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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