I Have been using my afk fish farm to get some enchanted books. and have been storing the excess in double chests. it happened when the latest string of snapshots came out i think it was 18w22a when it happened for the first time i went to my chests and found out that they had all changed to protection 1-5(yes i know it doesn't go to 5 but they were there) this is real frustrating especial cus it changed my all my books even the ones i enchanted my self