As far as I can tell, unlike RGB Smart Objects, CMYK Smart Objects do simply not function as expected (Edit: or rather as one might expect) in this regard.
Mabe you could file a bug report over at
Thank you for the quick response.
I have been trying out stuff right now and if I convert the group to a smart object inside the psd file - open the smart object - convert the group to another smart object and convert this to Adobe1998 - close and save it within the first smart object and close and save this smart object into the psd file - it works...back to 340% that was assigned to the original psd file and visually as before.
So agreed it does look like a bug or a non-expected behaviour of cmyk smart objects.
The CMYK-SO-inside-an-RGB-SO-inside-aCMYK-document workaround works, but it’s a bit of a file-bloater and the additional conversion can be detrimental to image quality (though probably imperceptibly in most realistic cases).