The bug

Villagers can get into a state where all there trades are locked.

Cases

Unloading villager during refresh

This bug happens when you unload the villager before it refreshed its trades.

How to reproduce

1. Spawn a villager
2. Get all items it wants for its first trades in your inventory. It is easier if you are in Creative to be able to clone items if you need more.
3. Start trading with the villager
4. Without closing the GUI use all trades until they are disabled
5. Close the GUI and directly afterwards close the world
6. Load the world again
   → The trades will still be disabled and won't be enabled again

In some cases, such as when trading with Leather Workers or other villagers with low numbers of total trades, it is possible to skip steps 5 and 6 as those villagers can have all trades become locked without closing the world.

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

The fields net.minecraft.entity.passive.EntityVillager.timeUntilReset and net.minecraft.entity.passive.EntityVillager.needsInitilization are not saved to NBT.

Suggested fix

See this comment.

Trade not being used for the first time and having bad luck

Based on 1.11.2 decompiled using MCP 9.35 rc1

As of 1.11.2 (and earlier versions) the method net.minecraft.entity.passive.EntityVillager.useRecipe(MerchantRecipe) only enables all disabled trades if a trade was used for the first time or with a one fifth chance. This means if you have bad luck, you can end up with all trades locked as well.

It is not known if this is intended or if the developers just did not want that trading any trade always enables all disabled trades, but forgot that it might happen that all trades get disabled.

Fix for affected villagers

This comment describes a fix for already affected villagers in case it is not intended that villagers with only disabled trades exist.

Remark: Consider adventure maps, where sometimes all trades are intentionally locked. Unlocking them when loading the villager might break the map, because players can get item they shouldn't have.