I saw this question some where, I think Reddit, but I can't find it. I have same issue. Please see the screenshot beneath. I've uploaded the PDF to mega.nz I'm using Adobe Acrobat Pro DC Version 2019.010.20064.
I'm trying to convert the book's default (C, M, Y, K) = (99%, 98%, 18%, 6%) (ie dark purple) to (90, 0, 90, 0) (green). After I click 'OK' and click 'Fix', Adobe Acrobat executes the fixup, but the color fails to change. Why?