If this target is different than what you have in Adobe Acrobat Pro (which shows the default Adobe CMYK target Coated Fogra 39), the Output Preview will show ad-hoc converted CMYK values, where e.g. Affinity apps by default include the target ICC in the PDF, which is basically not needed since everything is already in target color space. This could be the 'illustory' problem of having a device-CMYK PDF that is produced if you used 'PDF (press-ready)' and either had the images already in CMYK mode, or checked 'Convert image color spaces' export option under the 'More' PDF export option page.