Hello !
Use This command for give enchanted book Protection 5 ou more.
/give @p minecraft:enchanted_book{StoredEnchantments:[{id:0,lvl:5}]}]
Use anvils with this enchanted_book "modified".
We can not go beyond the enchantement level limit. with this enchanted_book.
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There is no relationship between the bug and I present the "duplicates".

Enchantments with level over max default value are not supported, you have such an enchantment.

But it is not an issue with the effect itself

It is an issue with an enchantment above maxlevel, so yes, it is a Won't Fix duplicate of unsupported values.