Uh oh, this is not good. I looked it over a bit more, there are minute amount of the damping fluid leaked outside capsule, I can see it because it's stuck between the transparent plastic and outer black shell.

So as far as I figure, the liquid contracted in cold, the seal wasn't able to hold the vacuum, and allowed air to leak in. When the liquid expanded again at room temperature, some of the fluid was expelled instead of air.

I wonder if this will happen again and again, and more air gets in, more liquid leaks out.