I use Adobe Acrobat to change colors in a PDF. Why does mrserge 'convert everything to CMYK' by using the 'Map, spot, process colors' Fixup? Dietrich von Seggern never does. He merely continues in RGB:
Your issue has to do with the color space. The box in question uses ICCbased RGB - but you are right: just decalibrating it does not suffice.
What you should do is first analyze the box in Preflight. You can do so using the "List page objects grouped by type of object" profile and you can limit that to one page.
The screenshot shows that the color of the box is 0.82/0.81/0.88 in RGB. You can now put that value into the Map colors fixup which needs to be set up for the accurate color space of the object.