Cannot save CMYK black to k100

Hi,

I have a svg barcode and also a pdf logo. Two different files from two different sources. Both hat back only components, which are CMYK.

When I open Illustrator and select all, then go to the colors and change to K100, all seems to work. Then I save (tested localy, on a served, same name and different name), close the file, reopen it, and the back is again CMYK.

I can add other compontent and those save, so it is just the color I might be using wrong somehow?

I have also tested with Versions 28.0, 28.7.3 and 29.1, so it must be something I am doing wrong. Is there some kind of "save black as cmyk in illustrator"?

Any ideas, what I could try?

Thanks!