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Convert Images to true black with filter: grayscale(100%)

I have a document containing spot colors and grayscale images. We print on a risograph printer, so there is no cmyk, only black and spot colors.

The spot colors are working with alternate color.

Grayscale images are also working if they are already black and white, using the "-prince-image-magic: ignore-icc-profile;" option.

However: Color Images should be converted to black and white as well. Using the css grayscale filter they look good in the document, but the separation preview in Acrobat always shows colors on the cmy plates as well.

Any ideas how to achieve a true black conversion of images in prince?

Thanks for any help.
Do you have an output intent color profile specified? You could try the prince-pdf-color-conversion property.
If you have a grey ICC profile, you can convert colours to that profile.
If you have a CMYK ICC profile, Prince doesn't know how to convert to CMYK using only the black channel.