Actually, a better question might be how you avoided the rich black using what you believe are the exact same settings over the past two years!!
Simply stated, if you do a PDF export and the CMYK content in your document is in a different CMYK color space than that specified for your output and your export setting call for converting colors to the output color space without the preserve CMYK values specified, there will definitely be a CMYK to C'M'Y'K' conversion occurring. This has been true since at least the first CS version of InDesign (InDesign 3).
The problem with CMYK to C'M'Y'K conversions is that they are a 4-3-4 conversion. The CMYK values are not directly converted to C'M'Y'K' but rather, converted first to the color management connection color space by use of the input profile and then from that color space back to the CMYK color space specified in your export settings. Such 4-3-4 conversions do not preserve pure colorant values. Thus you have the possibility of ending up with rich black if your content's color space and the output color space differ. The preserve numbers option was put in to effectively ignore CMYK to C'M'Y'K' conversion entirely and only deal with the conversion of RGB color spaces to the output color space. (Another technology, known a “device link profiles” can be used for CMYK to C'M'Y'K color conversions, preserving pure colorants, especially black. Unfortunately, InDesign doesn't support device link technologies!)
Then we must come back to why things supposed worked the last two years without specifying the “preserve values” option?!?!? One possibility is that previously your default CMYK color space exactly matched the output color space, thus bypassing any color conversion at all!
Thanks Dov, all that makes complete sense.
You're right, I have no idea how it's worked this long, typically the workflow has been to convert all images specifically to the output profile then export however lately (not in the last two days) I've been linking images of all sorts of profiles and converting on export.
I think I've nailed the problem, working back from what you suggested I discovered my colour settings had been reset so in fact I was working from one CMYK profile and exporting the PDF to another. Correcting my intent profile to match my output profile has fixed the problem. Exporting as expected.
Dov do you think I should still be running into problems I've been lucky to avoid?
There are always more problems to run into, for better or worse.
However, if you are meticulous about specification of color spaces, this particular problem should not occur.
In general, I try to avoid workflows in which graphical content is converted from its native RGB (Adobe RGB, sRGB, etc.) prior to the RIP process. I use CMYK for text and simple vector graphics for which I don't particularly care about the differences in how such content will be rendered by different CMYK output processes and export with the preserve numbers option which preserves the CMYK values in output color conversions (it effectively means that for CMYK only, color management conversions are not performed). My RGB imagery is passed through with ICC profiles to my PDF file.
Adobe most strongly recommends use of PDF/X-4 for PDF export since it maintains transparency and doesn't do any premature color conversions.