The excellent canon novel Lost Stars details the story of Thane Kyrell, who is initially stationed on the first Death Star before defecting after the destruction of Alderaan. Now, if I remember correctly, he was on the Death Star for at least a week when Alderaan is destroyed. However, where was he during the events of Rogue One, when the Death Star is used to kill thousands of people. I'm aware that the two materials were made at different times, but is there an in-universe explanation, or at least something from the Lucasfilm Story Group?
I don't think there is an official canon in-universe explanation for this, but after just finishing Lost Stars, I have 2 personal theories as to why (assuming he was still on the ship, and hadn't been transferred off)
While Thane Kyrell was an officer, he was only a Tie Pilot, so would not necessarily be involved in any of the day-to-day operations of the ship. And he certainly wouldn't have been on the bridge or in any kind of command center. When the initial "weapon test" on Jedha was conducted, the Tie Fighters were not scrambled, so he had no way to really witness the event first hand. On a ship as large as the Death Star, he may not have even know about it or only heard about it as a weapon test, or dismissed it as a rumor.
A second alternative explanation, assuming he did learn about the destruction of Jedha first hand (or believe the stories he may have heard in the officer's mess), that might have just been another reason for him to keep hating the Empire, but it took the destruction of Alderaan and all of the innocent lives that were killed that finally pushed him to defect.