PDF export making Pantone oranges look muddy, issue with x1a format

Hi there

I have a Illustrator file (working in CMYK mode) that features a gradient using Pantone 144C (orange). The document has an assigned colour profile of ISO Coated V2 (ECI) - which I have been advised is the preferred colour format at present for high quality printing, over FOGRA39.

When I export it for print, as a Illustrator PDF with no colour output settings changed, everything looks fine (though the PDF shows a Simulation Profile of being US SWOP Coated - not sure why it would have changed to that).

My preferred way of saving print ready PDFs is x1A, with the colour Output set to the working profile of ISO Coated V2 (ECI) - but when I do that, all the orange gradients become washed out and muddy. I tried playing around with the colour profile Outputs in x1A format, though nothing seems to help.

Any ideas as to why? It's driving me nuts. 

Many thanks

Karl