I’m not saying my experience necessarily adds to the analysis of the cause of the problem discussed above, but I have just experienced this same issue with my P600, and the fix outlined by Chris also worked for me (thanks Chris!).
In summary, I always do a nozzle check when I switch on, usually weekly, and for the 18 months I've had the printer, it has always been 100% OK. Today, though, a good 80% of the MK pattern was missing and after a cleaning cycle that made no difference, a bell rang in my head about this article, which I’d bookmarked when I first came across it, “just in case”. Replacing the yellow cartridge – which I’ve been expecting to have to do for some time – brought back the MK, just as the original post describes.
To be honest, I didn’t really read the discussion till after my problem was fixed, so I didn’t explore the alternatives put forward. However, to answer Chris’s question “Has anyone else faced that problem?”, I would say, yes, I have.
Cheers,
Bob