here is how i see your rectangle in the pdf in inkscape:

in the front you see the color dialog from scribus, with the color from the .sla:
- the colors seam to visually match
- when i look at the values, without doing the maths, the M and Y channels seem to be switched between inkscape and scribus.
- scribus shows the same color when opening the .sla and the .pdf
i'm trying with one of the inkscape 1.0 betas / alphas...
checking the same things on your computer might help you find a path to a solution...