Environment : Any Aruba OS and Any Aruba Controller/APs.
Client connectivity to 5GHz will be random. Some smartphones will not connect to APs on 5GHz. Same clients will connect to other APs with same configuration or even connect to the same APs they were not connect to earlier.
This depends on the channels supported on 5GHz by the client devices and the channel used by the AP for broadcast. When ARM selects a channel that clients support, they can connect else they cannot.
As shown above, one way to fix this is by enabling 802.11d on the "a" radio profile or to customize the AP regulatory domain profile and enable channels supported by all clients on the network.