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So this may be a silly question, but when they use the ability to drop spore mines in the movement phase, do they then have to fire on/assault that same target later in that turn if they decide to shoot/assault?

Nope. It is not shooting per se, and therefore does not bind the model to assault the unit(s) that get hit by it's spore mines.

Especially considering the fact that the Harpy can fire in the shooting phase and there is absolutely no basis to limit those shots to units hit by it's spore mines.

This is a specialty attack that occurs in the movement phase, and does not count as the Harpy's shooting.

That's how I read it, but since it doesn't come out and blatently say you can target another unit later, I wasn't sure if I could employ them in that manner.