Plein is a tricky word that can be a preposition, an adverb, an adjective or a substantive.
It is however clearly used as an adjective here so there is no reason for it not to agree with the name it applies to:
En pleins préparatifs is the correct form and the singular plein is a mistake in that position.
See http://alafortunedumot.blogs.lavoixdunord.fr/tag/en+plein
This mistake looks to be quite common and can be found even in printed books.
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