When you put an enchanted item in an anvil (lets say a book with power I) and the last item held was a book with power II. The power II book will change to a power I book. You have to rename the item or combine it with something for it to do this.
Linked issues
duplicates 1
Comments 5
I think what's happening is that it's getting the effect right, but consuming the first book in your inventory, which may not be the one you used.
Please comment any additional information on ticket MCPE-10281. Thanks!
I encountered a similar issue (if not the same) on 0.12.1 alpha / iOS. I had an unenchanted diamond pickaxe and two enchantment books, one having Unbreaking II and another having Silk Touch I. When I combined the pickaxe with the book of Silk Touch in an anvil, the combination itself succeeded but the book of Unbreaking in my inventory somehow transformed into a book of Silk Touch I.