In my case - piano, strings and organ sounds worked fine. That one electric piano patch just produced an output with one channel inverted. The 'fault' was revealed because on the M32 Midas, the gains and faders were linked so exactly the same gain and fader setting for both, plus identical sends to bus 3 from both of them. This caused the cancelation. a small change to gain, fader level or send level on just one channel of the pair produced sound. the link feature sat the Nord in exactly cancelled out status. I don't know if they have a feature to invert one of the channels in a particular patch, but it did. never had it before with a Nord, but one of those unlikely multiple cause conditions.
I have to admit that's a remarkable condition.
In sample based keyboards, I've come across comb-filtering and cancellations when stacking samples, and even modules, because, after all, they're using the same waveform. Seems surprising this hasn't come up during practice.
Channel inversion is a pretty ordinary control on a mixer, but not so common on a keyboard. Still in all, the fact that it CAN happen, in something as mature as the N4 is somewhat astonishing. They usually tailor the samples to expand the stereo field and thus they're not-identical.
I mean, you're right. Linked is linked. Identical is identical. The only other possibility is that you were somehow hooked-up with 2 channels of the same side and one was inverted. I'm sure it wasn't, and I'm sure that was checked, and highly unlikely, but it woulda produced the same result... EXCEPT it woulda been on every patch. Just EP? Gotta admit, it ain't the mixer.
It would be interesting to see what Nord says about this. I mean, if you could do this on stage, you could easily do this at home and make a vid/mp3 and send it to them. Wait, I can almost predict this. They'll harumph a bit and then say that OH, yes, this was cured in our latest sound set. Then ask if you're using the LATEST sound set from their download page? You're not? WELL... we can see that must be your problem.
Might be the weirdest keyboard thing I've heard of in some time.