I get the exact same problem. Acrobat USUALLY converts the file with no problem, but every so often I get a file that won't convert. What's frustrating is that it doesn't tell you WHY it didn't convert, so you have nothing to go on to try to fix the problem. I found a workaround. I create an Illustrator file, placing (but NOT embedding) each PDF page on a separate artboard. I then save the file as a PDF (still in color). Then, when I open that new PDF in Acrobat, it converts it to gray gamma, no problem. This is definitely a bug in Acrobat.
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