Is the switch port to which the AP is connected configured in Trunk mode, tagging VLAN 20?
If you configure a switch port in this VLAN and connect a machine, is the behavior the same? Don't forget that the Wireless network is just an extension of the wired network.
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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