Memory Alpha says something else entirely, but no sources are cited [
https://memory-alpha.fandom.com/wiki/Valeris]:
Saavik was to have reprised her role in Star Trek VI: The Undiscovered Country, but was replaced with Valeris, reportedly because Gene Roddenberry was concerned that fans had become so fond of Saavik that they would react negatively to her turning out to be a traitor. In addition, no one wanted to have to cast a third actress to play the same role: Kirstie Alley had declined to return, and Nicholas Meyer was not a fan of Robin Curtis's portrayal.
They, too, say "reportedly." It would be interesting to know the truth.