Enchantments on enchanted books change when the world is exited and reentered. Happens when exiting to title screen and sometimes when going to and back from the Nether. 

How to replicate:

1) Create a new singleplayer Creative world in 18w22c

2) Place down an empty chest 

3) Put an enchanted book inside (one that isn't a Protection book); tested with Silk Touch

4) Quit the world to title screen 

5) Enter the world

6) Open the chest 

The enchanted book has probably changed its enchantment from whatever it originally was to a Protection enchantment.