I am preparing a large document that will be printed in one colour, and I tried including a few images in png-format that have been converted to a greyscale colorspace (DeviceGray). In the resulting pdf, if I have a close look at it, it seems like there is a "pdf-container" that Adobe Acrobat Pro sees as having a "Blended sRGB colorspace", and that container in turn contains the actual image, in its DeviceGray colorspace. I don't know enough of the inner workings of the pdf format to understand exactly what happens.
The problem is that the publishing house I am working with is unable to process the resulting file, since apparently the Blended sRGB colorspace somehow bleeds into the whole page, resulting in a non-printable file (and their prepress software is unable to fix this).
I have solved this by converting the actual images to greyscale jpg instead, and everything is fine. (Importing an image converted to greyscale pdf works too.) In these cases, there is no intermediate container in the pdf file, but this seems like a kludgey solution.
Why does this happen? I assume it is a bug somewhere. I am typesetting with pdfTeX 3.14159265-2.6-1.40.19 (TeX Live 2018) and including the files with