What export setting are you using? It looks like your RGB "grayscale" is getting converted to CMYK on the export.
Hey thanks, I don't know much about it but I simply chaged the image color settings right clicking and changed it to srgb, and that fixed it.
With which program do you view the PDF? Anything else than Adobe Acrobat is not reliable.
What is the color space (mode + profile) of your document?