Other enchantable items produced by crafting behave as expected, and can be combined in anvils as expected.
Crafted enchanted books can be further enchanted by legitimate books in anvil, displaying both names, but when the merged book is used resulting book is used to enchant gear, the crafted enchantment disappears.
Enchanted books given using the command "/give @p minecraft:enchanted_book[minecraft:enchantments={mending:1}]" display similar bugged behavior.
Further investigation:
Other enchantable items produced by crafting behave as expected, and can be combined in anvils as expected.
Crafted enchanted books can be further enchanted by legitimate books in anvil, displaying both names, but when the merged book is used resulting book is used to enchant gear, the crafted enchantment disappears.
Enchanted books given using the command "/give @p minecraft:enchanted_book[minecraft:enchantments={mending:1}]" display similar bugged behavior.