I've experienced the same, and have my eyes open for some glass dropper bottles to see if that might make a difference.
Odd timing of this post, I just grabbed my plastic dropper bottle of acetone yesterday and noticed the bottle crazed and empty.
If anyone can confirm that a glass dropper bottle works, that'd be great. I wonder if the solvent fumes attack the rubber squeezy thing? I'd think yes? The bad thing about ordering glass is the weight. Shipping may exceed the cost of the bottles.