P: A vector shape with feathered mask behaves weirdly when duplicated (CS5)

Here's a very weird problem.

Create vector shape, add feather using the Mask palette. So far everything is well.

Now duplicate layer using Cmd+J or Alt+drag. Duplicate once or twice. Move layers around.

Now layers are rendered incorrectly. Some appear corrupted, some are invisible, some are rendered on places on the canvas where it isn't supposed to be (not where the shape is actually located).

The only way to fix this is to readjust the Feather on the Mask palette (if you can find the misbehaving layer) or close and re-open document.

Reproducible every time. Tried it out on different computers with different OS's with different versions of Photoshop. Trashed preferences, trashed all fonts, even re-installed OS didn't help.

It is driving me INSANE.

Would love some help on this, as it is preventing me from doing my job in a timely manner.

Thanks!