The silicone is free from the core and the air inside the toy heats up and expands.
The Leaf toys have the exact same thing. It's because they're waterproof.
I don't know how they would "solve" that unless they took away complete waterproofing to allow a vent for the hot air. People have solved this with a tiny pin-hole, so if someone's *doesn't* do this, then they have a "leak" in the waterproofing at least that size to allow air-flow. I doubt it would compromise waterproofing to a huge extent, so they can probably still label it as such if that's the case.
It's not gross, it's just a physical property of hot air. :/
I wouldn't return the toy because of this, it's not really a defect and you're likely going to get a toy that does the same thing. I guess it just feels like a waste to me since the toy can't be re-sold and nothing was wrong with it.