Villager change his trade even though, it was locked

​So I was playing on my survival world and I finished building my Village. Then I saw a zombie librarian which started burning, cause it was morning and died.

Then I waited for new one to grow. After that I started to rolling his trades until I got my Unbreaking III (it took me 3 hours). After that I ran into my house to get paper to lock his trade and I did that.

So I was really happy about it, finally had my mending and Unbreaking III villager. But then I looked at his trade again and I saw he was locked bud he sells BOOKSHELVES INSTEAD OF UNBREAKING III.

So please fix this mojang