TL;DR: The quest "Poseidon Adventure!" seems to be bugged, as it doesn't advance past "Defeat Sand Squid Tentacle".
I'm putting this topic out there to see if anyone is running into the same problem, or a similar one elsewhere. I've had the quest Poseidon Adventure! for a while, the one that requires you to beat the Sand Squid hidden boss for Atlantea. After assembling a team and heading straight to the area, we defeated the boss in one hit. However, none of us had the quest advance as it should. We attempted the boss again, this time defeating the main bosses first and the minions after. Unfortunately, the quest still did not change for us. We received loot each time, and had the instance quest complete both times. I'm not sure if anyone else has a similar experience or problem with this quest, but either we're missing something or this has to be bugged.
This just happened to me twice. I did it with a maxed friend first who never did the quest before and we both couldnt advance (i fled once so we thought that was it). Then i tried it again with a team, we did the dungeon first this time and the sand squid second and we had a good strategy so that no one had to flee in case that had to do with anything, but then again we couldn't progress after defeating the sand squid. My membership is about to end and i really dont want to do it a third time.
I just tried this quest and it didn't count. So, I came searching for answers and I can confirm that this quest definitely gets stuck at kill the sand squid and will not continue. Please fix this, thanks!
This problem still is happening. I ran the sand squid instance at least 6 times and still no credit. I've reported it to kings isle so they are aware. Not sure what they're doing about it though.