Is it not that the FAA advice is that one is susceptible to decompression sickness above 18,000ft, regardless of whether or not you are breathing oxygen?
It could well be among the volumes of "advice", though I haven't seen this one. I cannot really see the mechanism though. Also, when breathing oxygen, one is invariably flying a non-pressurised plane so a decompression is not likely.
But this stuff varies with individual physiology anyway.
There is no doubt cannulas do work just fine beyond 18k and one American mag did a test up to IIRC 25k and found them fine. The problem, which I have found myself, is as I said above: if your breathing gets lazy then you could have a problem and that problem (loss of useful consciousness and thus the ability to do something about it) rapidly gets worse as the altitude goes beyond around 18k.