I haven't heard of yolk sacs retracting, but it may be possible as long as the yolk wasn't ruptured. But the same advice applies. This could happen again. The chick must be watched. If it does happen again, and you can determine it's the yolk sac, which would be obvious if you can see the little...
What you saw may not have been egg sac prolapse. It may have been vent prolapse. Judging from the photo in your first post, it's the right color. Yolk prolapse probably would be more yellow.
Vent prolapses often retract on their own, so this is the explanation I would offer to explain this...