The below is from ZD User Guide.
ZoneDirector comes with a built-in DHCP server that you can enable to assign IP
addresses to devices that are connected to it. ZoneDirector’s DHCP server will only
assign addresses to devices that are on its own subnet and part of the same VLAN.
Please use the below link to download the User Guide, and refer page 76.https://support.ruckuswireless.com/documents/1106-zonedirector-9-13-ga-user-guide
I am not sure about the configurations on the core switch, if you have connected these APs on core switch then VLAN interface would the reason for AP/ZD communication.
In this case AP, ZD and clients would get IP addresses from VLAN 3 (ZD VLAN), this is because of above reason.
I hope this answers your questions.
I would recommend you to get on call with Tech-Support; who should be able to provide more information regarding this.