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The head and body was made of two different plastics. that may be the problem.
It's a chemical break-down of the plastic that causes the body to be hard. It's a common problem with scented ponies, since more was added to the plastic in the original manufacturing process. There's no way to fix it, sadly.
I think the type/amount of plasticiser varied between batches of vinyl. If you pop her head off is the body sticky inside? I find that's the case with my harder ponies that aren't weeping plasticiser on the outside, it's weeping on the inside instead.
There's not much to be done except keep an eye out for a Raspberry Jam with the opposite problem so you can swap heads, and make sure your display and storage areas are climate controlled to minimize further degradation of the plastic.