PDF/X has a series of printing related requirements which do not apply to standard PDF files. PDF/X is often necessary if you want to produce a master for an online-printery.
feature request. status NEW.
The underlaying CMYK color space needed for PDF/X ISO 15930 is not natively supported within LibreOffice. We only have limited pdfium based import filtering and convert to RGB (sRGB) but not the other way for export filters. The UI provides for color input as CMYK value--but that is converted to RGB.
Better bet is export to PNG and find other conversion to CMYK based PDF there. And for printing process separation, we now support the "freecolor HLC" based color palette, which will render with good fidelity back to CMYK color process separations.
To me, a pretty clear WONTFIX as native CMYK support is out of scope.