Hello ,
Thanks for your advices! It's helpful.
My IAPs are two 305 and one 315 with firmware 8.6.0.5_75979 released in Y2020.
It's still the same situation after three years. Guest wifi network with "Virtual Controller managed" Client IP assignment, and "Custom" Client VLAN assignment with my customized local DHCP scope.
Wifi clients won't get DHCP ip successfully if they connect to non-VirtualController AP. Only AP plays as VC role could assign ip.
If I take your workaround to turn Client VLAN assignment from "custom" to "Default" with settings for "Virtual Controller Assigned Networks - Default DHCP Scope". Then wifi clients can connect to guest Wifi successfully after an IP is assigned.
For Aruba , please take into account this important DHCP feature (or bug). It has been for several but no improvement yet.