Just running the heroic mission on Titan & just completing it does not start the 'Outbreak prime quest'. It seems so far that the people who have no clue why people are 'locked' out of an 'unlocked' room is due to some players actually started/finished their 'Rat king quest' while others (a decent mount) just dismantled it; thus never starting it. The Outbreak prime quest is an option through that Heroic adventure only if you have completed past 'Enemy of my enemy' quest on the Rat King quest line.

By just run the mission I am referring to Enemy of my enemy, if you dismantled it without even starting it, it will be at sloan.

Yes as I posted above that the mission 'Enemy of my enemy' is not on some peoples 'maps' inherently due to it sitting in a quest token on a vendor called Sloane on the planet Titan. Not many or the majority of the people who never started it know about this. Maybe Bungie actually planned for this mission 'Enemy of my enemy' to technically lock out players from starting the OB prime quest via that heroic adventure and really want more people to go get the Rat king ... or this is a logical error and could be adjusted with a patch to not allow the completion status of 'Enemy of my enemy' to stop players from starting the OB prime quest line.