Upon completing the quest, "The Stone Circle", Marvon Rivetseeker in Tanaris, should give two quests, both of which go to Sunken Temple:
http://wowwiki.wikia.com/wiki/Marvon_Rivetseeker
Yet, today that I turned in the pre-requirement, I was only able to pick up one of the quests. "Into the Depths", while the "Secret of the Circle" is nowhere to be found.
I went to the bugtracker and search for anything related to "Secret of the Circle" yet my search finds no results, so I wonder if I'm the only one having this problem.
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Edit:
Changing my bugtracker search parameters to look for Marvon Rivetseeker, I have found: https://report.nostalrius.org/plugins/tracker/?aid=2248
The report is however the opposite of my problem. This person was capable of picking up "Secret of the Circle" but couldn't take "Into the Depths".
He did not specify what his faction was, but my gut tells me that he might be Horde, and that the quest database is bugged to offer only one of the follow-up quests to each faction. In fact, both quests being neutral, should be available to both factions.