When you try to give a crossbow mulitshot either by command or book you can't. Its considered a trident enchantment and you can put it on a trident even though it throws 1 trident.
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Is your crossbow already enchanted with Piercing? Multishot and Piercing cannot be combined.
Additionally in Creative mode you can enchant items which you cannot enchant in Survival, see MC-95555.
I cannot reproduce.