The only thing I can think of is maybe theres a higher ilvl requirement to do a random (heroic or not) than to just warp to the instance... so even though you can go to both, having discovered them, youre only good enough for one of them. Just spitballing.

I have discovered throne of the tides. I can go to it through Specific Dungeon, but if i queue for a random dungeon with my group it says "one or more of your party does not meet the requirement".. would be nice if they told me what req i wasnt meeting.