1) JPEG CMYK is converts to RGB 2) PDF/A-1a and PDF/A-1b doesn't support transparent content. 3) Libre Office don't show (5.4x) or show with bad position and bad size (6.0.x) if EMF file don't contain EMR_SETMAPMODE.
PDF, PDF/A and JPEG-CMYK
XFRX converts JPEG-CMYK to BMP since XFRX 14.6. XFRX has full support for JEPG-CMYK since version 18.1. But, XFRX converts JPEG-CMYK to BMP if output is PDF/A.
JPEG CMYK contains ICC profile. Output PDF file will be greather than PDF file generated in previews versions.