I will share the latest info.
The documentation now clearly states that LACP and Link Aggregation are not supported on Meraki MX.
MX Layer 2 Functionality - Cisco Meraki
https://documentation.meraki.com/MX/Networks_and_Routing/MX_Layer_2_Functionality
> LACP & Link Aggregation
> The MX does not run LACP or any link aggregation protocols. Connecting aggregated ports to the LAN of the MX is not supported; all connected ports should be un-aggregated. If multiple ports are connected to the MX from a single switch for redundancy, it is highly recommended that you run STP on that switch, to ensure that one of the redundant ports is safely put into a blocking state.