I am having the same problem. Disconnected and started running a file check after the boss not appearing twice.
After the file check, I decided to try a third time to find the boss once again did not appear for me. It was a team of 7 out of 8. Two of us including myself could not see the boss, leaving five players to fight and damage the boss. My Photon Blast Gauge kept climbing up even though I was attacking the air. The boss died in 9 - 10 minutes....(whishc should be an impossibility) with the party claiming that it was stagged frequently and for too long.
I did not get any loot from the boss, but I did get the weekly clear for Urgent Quest.
I could not return to the field and had to "Abandon Quest" which put me into an infinite telepipe loop. Had to force-quit via the task manager. I think I did get EXP for the fight...