Hi craig,
Yes they do, though we didn't see it. In our group a few suffered symptoms, some serious and some even at 2,600. The worse i got was a thumping headache.
We left 4.00 am for the Thorung La pass getting there at 10.00. The celebrations were very premature as the 5 hour descend was on a steep ice 90% of the time, which killed my butt, knees and confidence. The advantage of doing in Dec was that there was hardly any one there.
Cheers
ps. no doubt you found it awesome too.