I have found at least a dozen caches above 1000 m, in Norway, Finland and on Iceland. But the highest listed is the higest elevation cache I found in the US, at just above 500 m.
I have checked the list for some of my friends, and they also lack their highest elevation caches. The only exception I found is one who had all his highest caches in the US.
The caches are not missing, e.g. my first FTF was 1200 m up,and it still is there. It is just that the high elevation list is wrong.
Looks like a bug.