Have you used an OXO measuring cup? They really are a lot easier to use when baking and cooking, and my sponges never have a problem getting in those corners.
On the other hand, my "pyrex" measuring cup lost all of its markings to the dishwasher, which also makes it a bit difficult to measure anything in it (without a scale).
This happened to mine as well, I was a bit disappointed, expect OXO to be better quality. They'd probably replace it if I asked but I feel like it's my fault since I dropped it, even though a plastic cup should survive a fall on the kitchen floor.
I think the joint is somewhat plastic, to the point that it feels slightly flimsy, possibly to prevent drop failure.