No man can answer your question.

https://m.xkcd.com/1704/

This is a neologism to describe something that is "not in the DSM" and not concretely documented in a particular study. This alleged phenomenon and coinage is the emergent result of some analysis by this journalist, writing for a commercial news site that sells ads but does not launch any astronauts or spacecraft anywhere, much less interview or evaluate them once they've landed and arrived home.

Commenters who are comparing it to a selection in the DSM-IV, or "oh yeah, I felt like this once" anecdotes are projecting subjective experience onto subjective journalism about subjective testimony. Where is our scientific empiricism? Would you like some ketamine? Would you care to take less ketamine?

Therefore, "observer sensation" means just what the journalist chooses it to mean. Nothing more, and nothing less.