A quite late reply, is this still an issue or was it solved? I checked the database and it's at 21 meters.
Theoretically a geocache can disappear from Project-GC's database for various reasons. For example it disappears from Geocaching.com, or their API just gives an invalid response that it doesn't exist (it happens). In those cases the geocache will be added again, and will get an elevation of NULL. But that should be corrected within minutes, normally.
Couple of weeks ago I fixed my script to skip caches without elevation data because one cache did not have it and the script crashed with user who has found that cache. In that case the problem was existing probably some days.