Forgive me for going through this again, but I want to make sure we are discussing and understanding each of the key pieces. So let's walk through this step-by-step to try to decipher your problem.
When an AP boots, it needs 6 pieces of information:
IP Address
Subnet Mask
Default Gateway
AP Name
AP Group
IP address of the controller the AP will initial communicate with
IP address/subnet mask/default gateway are typically received from DHCP (let's make sure the AP is getting all three pieces). The name of a new AP will be the MAC of the Eth 0 port on the AP, the group will be 'default'.
Once those 5 pieces are obtained or set, the initial controller is obtained in the following order
statically configured
DHCP option 43/60
Aruba Discovery Protocol (ADP) multicast and broadcast
DNS
The initial controller does not have to be the Master controller. It can be any controller. This variable dates back to ArubaOS 2.x when it did have to be the Master, now it is just the value of the controller where the AP will initially communicate with to download it's LMS-IP address. If you set this value manually on the AP, the command is "setenv master x.x.x.x" , (x.x.x.x = IP address of a controller) but even though the variable name is master, it can point to a local controller. If "setenv master" is not set, then the other methods are used. DHCP option 43/60. DHCP option 60 should be set to "ArubaAP". If DHCP option 60 is not specified then the AP will send multicast requests (IP 239.0.82.11) and broadcasts to find a controller , on the VLAN the the AP has an IP address on.
You can console into an AP and intercept and stop the boot, go into the apboot> prompt and type "dchp" to trigger a DHCP and see what I address the AP gets. Also from a console, if the AP is at the ~# prompt, you can type "ipconfig" to see what address the AP has received after it has gone through the boot process. At the ~# prompt you can also use ping to see if the AP can ping devices on the VLAN, including the local controller.
After the AP boots and gets all of this information, the initial controller, the one that it discovers or is pointed to, is used to provide the AP with the LMS-IP address, the LMS-IP is the address of the controller where the AP will download it's config from and then terminate it's GRE tunnel to. So the AP communicates to the LMS-IP controller, downloads it's config, and terminates it's GRE.
Here is a good link that explains some of this.
https://www.flomain.de/2018/02/unified-aruba-controller-discovery/
I hope this helps,