Enchantments only target something on initial cast, so if a creature were already enchanted and then later gained protection, the enchantment would fall off. (Much more rarely, it's also possible to use enchantment swapping or other shenanigans to simply have an enchantment put onto something without targeting it, which protection would also make immediately fall off, but that's much less relevant than this thing being enchanted and then getting leveled up to 8).

That's the reason that [[White Ward]] has to have its second line. It would be perfectly fine to cast it, and the targeting would be over by the time it gained protection, but then the protection would make it promptly fall off otherwise.