Reported here:
viewtopic.php?f=47&t=53905They attempted a fix in the latest patch:
jorb wrote:Key Fixes-----------------------
- May have fixed the bug by which certain objects would sometimes drop their variable materials on load, or temporarily. Can't actually reproduce the problem, so please verify that you can't either. Requires client update.
Are you using a client that has received an update since the latest patch?
In any event, it's a temporary glitch and may or may not go away upon reloading the map. The actual object on the server isn't affected.