It seems to be a common property of polymers. Here's a description of the mechanism:
So that plastic bag may be permeable to water vapour, and to a lesser extent to liquid water. An important function of the bag is to stop the solid desiccant mixing with your vitamins. Maybe the reason it's so resistant to tearing is because the pellets are harmful if swallowed.
I've only made 1 balance staff so far.
I've got a German aircraft clock that has a broken endcap jewel that also had both ends of the pivots worn flat. I've rounded them again and once I've received the new reamers for my jeweling tool and fitted the new endstone I will find out if I will be making my second balance staff as it may now have too much endshake.