movieClip._visible not working as expected [Flash Pro 8]

I have some AS that creates a movie clip and later tries to make it go away. However, _visible isn’t working as I would think. Here’s a sample:

I create the triangle:

 triangle30 = createEmptyMovieClip("triangle30", this.getNextHighestDepth());
 with(triangle30) {
  lineStyle(1, 0xff0000, 100);

and then later try to make it go away:

triangle30._visible = false;

It doesn’t work, however. So, I tried trace(triangle30._visible) both before and after the second chunk of code, and it does indeed change from true to false. However, the movie clip is still visible on the screen. Any idea what’s going on?