It's a little difficult to tell due to sunlight, but the split actually starts at one of the holes. It moves diagonally at the angle of the hole for a ways, then turns more vertically.
The split stops about where the grip rings start. It does go into the first grip ring but not all the way through that grip ring, if that makes sense.
And this is me gently squeezing the cup to show the two sides:
My best guess is that the cup split while I was folding it for removal. I've had this cup since the summer, so I'm surprised to have this issue now -- after I've had several successful runs with the cup but before it's anything close to "old."
I did contact Fleurcup's customer service. They said this was the first they'd heard of such a problem and suggested that maybe I had long nails that pinched the cup? (I do not, nor do I use my nails to remove my cups at all.)