She also appeared in several episodes of Fame, as Doris' mother.Those years are somewhat later than TWOK's release in 1982. But she did appear in a Fantasy Island episode in '82, so maybe she and Montalban had a chance to talk about it.
The point being that if she was working up until 1996, and well enough to be a TV series regular, she was also well enough and still working at the time TWoK was being made. I don't know why you seem to think my mentioning later dates negated my point, when it in fact made my point.
But the demands of a movie and a TV show are different, and it's possible that she/her doctor felt that a movie would be too taxing for her. I read something about this many years ago, so am possibly misremembering it.
That is a possibility.