When a group policy is applied to a VLAN, that policy becomes the new "network default" for any other group policies applied to clients in that VLAN. Since this policy is the new "network default," the client devices will still show a "normal" policy applied under Network-wide > Monitor > Clients.
For example, a group policy named "Guest Network" with more restrictive layer 3 firewall rules than the network-wide configuration is applied to the guest VLAN, and a second group policy "Low Bandwidth" has a custom bandwidth limit, but is set to Use network firewall & shaping rules. If the Low Bandwidth group policy is applied to a client on the guest VLAN, the client will use the layer 3 firewall rules configured on the Guest Network group policy, not the network-wide layer 3 firewall rules configured on the Security & SD-WAN > Configure > Firewall page.
https://documentation.meraki.com/General_Administration/Cross-Platform_Content/Creating_and_Applying...
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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