I was trying to get my villagers prices lower, so I got a zombie to kill it, and then i cured it and it worked perfectly normal. I then tried to cure the same villager around 10 minutes later as the price was not low enough, but when the zombie killed the villager, instead of turning into a zombie villager, the villager just died. I then proceeded to get a new villager with the same book and tried to cure it, the same thing happened, it just died. I know this must be a bug and not something to do with my survival world or something, because after that I went into a creative world to test it out and the same thing happened unless I just cured a zombie villager.