Just found this in "Colour Photography" by Eric de Marie (1968), p.110.
"Except for the polarising filter, the effects of a filter on a film are not those you would see if placed the same filter over the viewer. The filter would not modify the highlights on the film but would only begin to show results, and with increasing strength, as the tones darken. In a snow scene photographed with a pale yellow filter, for example, the highlights of the snow would remain white on the film, but the middle tones might look slightly yellow. A filter always has more effect, therefore, on an under-exposed shot than on a correctly exposed or over-exposed shot".
It's clear that he's talking about slide film, although it's not specified (later on p.115 he recommends filters for negative films to bring the color temp back to daylight and reduce amount of adjustment while printing)
So, on a negative film, the filter would have more effect on highlights and midtones, and virtually no effect on shadows, more effect on overexposed film and less on underexposed.
If that's correct, it's quite a big difference between the effects of filters on slide and negative films, and the difference in applications.
For example "correcting" bluish shadows with the same number warming filter will have more effect on slide film than on neg film.
Does this sound right to any of the filter users here?