Nighthawk M2 access control whitelist

Hello

I have a Nighthawk M2 mobile router. When the access control whitelist is enabled I am unable to add more than 15 devices to the list. Attempting to add more shows "Error adding entry" and I have to delete an entry before a new one is allowed. Is this a bug in the firmware or normal behaviour? I think this model of router should handle more than 15 devices.

Current firmware is showing as ntgx24_10.17.08.00.