Other mistake: After helping Longshot in the alley, Jubilee tells Logan "Are you blind? He's on our side!" to which Logan responds "We don't know that." This dialogue is repeated verbatim when the Professor meets Longshot. In addition to being obviously re-used vocal lines, the context of Jubilee asking "Are you blind?" doesn't make much sense the second time.
Continuity mistake: When Xavier is scanning Longshot's mind, the sides of Rogue's tights are yellow when they should be green. (00:10:00)
Continuity mistake: When Longshot is getting ready to throw his bolas you can see that he has five digits on each hand. In the next shot he is back to having his normal four digits. (00:02:10)
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.