Convert colours with 'dot gain (15%)' results in (very) low quality images

I have converted a PDF file with all sorts of colour images (spot/cmyk) - one is 100%K - to grayscale using 'Print Production' -> 'Convert colours' -> 'Conversion Profile: Dot Gain 15%'. The result is a deterioration in image quality, particularly the image that is 100%K and is in an 'unsupported colour space' that can't be opened with Photoshop. If I follow the same steps with a RGB PDF, the aformentioned image is just fine (eventough it's 95%K, but seems logical converting from RGB).

If I convert the CMYK PDF with 'Print Production' -> 'Preflight' -> 'Convert to grayscale', image quality is fine but that one image is no longer 100%K, but 95%K.

What am I doing wrong ?