If you activate and then have that Avatar ride in a rocket to a platform, you still see the button saying "Drop to planet."

If you disconnect from the Avatar and connect again, that button is gone. I tested sending a 2nd Avatar and returning it, but the 1st was still stuck. I also tested sending my character and returning, but that also failed to do anything.

If you delete the platform, you can continue to connect to it for the 5 minute grace period to restore platforms. After it, you get an error saying "Some avatars were not found when creating the list..."