I think of it a little like the same way a Shelmet has to lose its shell before it can evolve. There's precedent for Pokémon leaving behind parts of their old form upon changing to a new one.
Yamask and Phantump I'm less sure of. It may be an urban legend which gained traction, or they may be able to reproduce via multiple methods. That would be my guess: a Phantump can be bred and hatched from an egg or created spontaneously from a possessed stump.
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Yamask and Phantump I'm less sure of. It may be an urban legend which gained traction, or they may be able to reproduce via multiple methods. That would be my guess: a Phantump can be bred and hatched from an egg or created spontaneously from a possessed stump.