This issue does not occur with an untagged VLAN interface for management. To change the AP configuration, you must reset the AP device and allow the Gateway Wireless Controller to discover it and apply the new configuration. For instructions, see Reset a WatchGuard AP.
If an untagged VLAN is not a good long-term solution for your network, you can downgrade the AP device to v8.6.0-646. For more information on how to manage which firmware your AP devices use, see Update AP Firmware on the Gateway Wireless Controller.