I combined an Enchanted Book that had Power IV and Efficiency IV with an axe that had Unbreaking III and Efficiency IV. I expected to receive an axe with Unbreaking III and Efficiency V. What I received instead was an axe with Unbreaking III, Efficiency V, and Power IV. If my memory serves correctly, in 1.10 (and presumably previous versions), you could add enchantments to non-corresponding tools, it just wouldn't show up on the tool post-enchantment.
Linked issues
duplicates 1
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Like I said, it's not that the books themselves have conflicting enchantments. It's that it's putting those enchantments on conflicting items. For example, I now have an axe with Blast Protection IV. I believe, then, that this is a different issue that the one to which you linked.
Oh yes, you're actually right. But I suppose you're in Creative mode? If yes, this is the relevant report: MC-107905.
Duplicate of MC-30767.