Are you sure the uplinks between MXs, switches, and APs are all configured as trunks, tagging all VLANs?
The DHCP service is very basic and simple; there's no secret to it.
If I were you, I would double-check to ensure the trunks are configured correctly.
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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