It is highly unlikely that anything that transpired in the "Throne Room"? of the 2nd Death Star is known to any imperials. Only Luke, Vader and Sidious were there and two of them died, leaving Luke the lone survivor. In addition, Luke probably would have to fight his way out towards an escape ship before the 2nd Death Star exploded, which wouldn’t leave any 'witnesses' around that could investigate before the 2nd Death Star exploded.
In the book 'Aftermath' holo vids are circulated that show the explosion of the 2nd Death Star and both Imperials and Rebels around the galaxy are discussing whether or not the Emperor and Vader died and no one is sure what exactly happened.
From the book's prelude:
Admiral Ackbar: Today is a day of celebration. We have triumphed over villainy and oppression and have given our Alliance — and the galaxy beyond it — a chance to breathe and cheer for the progress in reclaiming our freedom from an Empire that robbed us of it. We have reports from Commander Skywalker that Emperor Palpatine is dead, and his enforcer, Darth Vader, with him.
Furthermore, without spoiling anything, a number of Imperial higher ups are trying to figure out what to do next and there is absolutely no mention of Vader's betrayal despite the topic of his death coming up a number of times.