Sorry if this is a duplicate, I've googled around trying to find information on it.

About half the time when I try to move the cone end of the support it just vanishes leaving the shaft.  I can move the joint where it meets the shaft up and down just fine, but the moment I try to move the cone, it disappears.  Undo might bring it back but often doesn't and instead leaves the tipless support and removes a different support.