Yes, this has been brought up before, but things happened quite differently for Valtane in STVI, than in 'Flashback'...
It was cool seeing Sulu, Kang, and Rand...but why did they have to kill off Valtane? (Especially since he was alive and well, minutes after that happened in the film when Sulu finds out about the Klingon homeworld; and towards the end of the film when we see the bridge crew of the Excelsior when Sulu is speaking with speaking with Kirk Enterprise).
From what I understand, the aliens of that particular had something in Tuvok that altered his memory, which is why we see a different version of Valtane's fate...i.e. the fate that didn't happen.
Well, at least both instances are 'canon'...so we see what DID happen (the film), and what DIDN'T/COULD'VE happened (the episode).
It was cool seeing Sulu, Kang, and Rand...but why did they have to kill off Valtane? (Especially since he was alive and well, minutes after that happened in the film when Sulu finds out about the Klingon homeworld; and towards the end of the film when we see the bridge crew of the Excelsior when Sulu is speaking with speaking with Kirk Enterprise).
From what I understand, the aliens of that particular had something in Tuvok that altered his memory, which is why we see a different version of Valtane's fate...i.e. the fate that didn't happen.

Well, at least both instances are 'canon'...so we see what DID happen (the film), and what DIDN'T/COULD'VE happened (the episode).