To be honest, I don’t know really know how to make it stick, I do have another option though.
It’s not actually natural for a voice to be hanging onto a note perfectly, especially for that long. I can’t pretend to know the exact details of your situation, but it is something most people seem to overlook. Instead of trying to eliminate the human element, maybe you should add a little more. Add a little vibrato, automate it to taste. That way its possible that your issue may actually become part of its strength.
Unless of course its actually tuning to another unwanted note. In which case try eliminating it from selected scale. If thats a problem, try automating it so it only changes temporarily.
Oh wait, hang on… Is this Waves Tune or Waves Tune RT. Because I don’t think you can automate the former.