ARP request that is made upon failover is not propagating along the network due to a firewall setting. This made all servers that are not in the same subnet (including Spotlight) unable to connect until the ARP entry in the networking device is updated.
Given this issue, the only node that Spotlight can detect in the list of servers for the Availability Group is the one that held the MAC address that the firewall has recorded as the listener MAC address.
References:
1. https://social.msdn.microsoft.com/Forums/en-US/2880180d-56cc-4b91-ba94-1cd5f06c7b20/sql-2012-availability-group-failover-not-sending-gratuitous-arp?forum=sqldisasterrecovery
2. https://support.microsoft.com/en-us/kb/244331
