The Dark Phoenix (3): The Dark Phoenix - S3-E13
Other mistake: When Professor X says "We must do all we can to defeat the Phoenix within her," there is a black spot that moves vertically across the screen as the camera is panning from Cyclops to Wolverine. It's as if there was a small bug on the lens. (00:11:00)
Answer: Chris Potter, the original Gambit voice actor, did indeed quit the role in the 4th season. His last episode was The Phalanx Covenant Part 1 so it seems reasonable that Fox would limit the use of the character even though it was recast. It doesn't appear that Potter left the role due to animosity, he stated in an interview that he wished to play Gambit in the first live action X-Men film.
BaconIsMyBFF