So I think I can confirm the chemical breakdown of the star posts when using the Amazon clear rubber washers. Here are examples of my star posts. The splintered one was taking direct ball impact at the top of the left sling. More interestingly, the other one which is cracked throughout, was in the return lane of that same sling.
There is no way that the second one was being hit hard enough to cause the kind of “shattered” plastic I see. Furthermore, all of the posts which had these washers now permanently have the washers attached to the bottom. If I go to remove them, they just shatter.
I am now testing clear plastic washers from PinballLife.