So I take a crossbow and I load an arrow into it. The crossbow has Multishot on it. Then, I disenchant the loaded crossbow. When I fire the crossbow, it still shoots 3 arrows from the disenchanted crossbow.

Also, if you enchant a loaded unenchanted crossbow with Multishot, and fire the crossbow, it only shoots one arrow from the crossbow while it's enchanted with Multishot.

This was done on an unmodified Minecraft 1.19 singleplayer world.