Don't drop the mission if you're bugged. It will just bug for you in the same spot. Instead beam up and either 1. travel to Nopada in Psi Velorum and get the mission complete but miss a piece of plot or 2. wait for a fix if you want to see the plot development before continuing.
I can confirm that Sela was not visible on a subsequent playthrough though she was "there" and the cut scene played out as if she was. In any case, beaming up and traveling to Nopada will allow you to complete the mission once you reach the stuck platform stage, although you miss some explanatory plot.
It's not that Sela doesn't spawn or disappears. She runs back into the previous room and you're supposed to go talk to her to advance (you can see her indicator on the map). But the door between you is locked so you're stuck on the platform.