Villagers with custom trades on a multiplayer server, spawned via commands, sometimes do not register a trade to become successful, despite having the correct input. This event is inconsistent, but happens more often when the input also requires custom items, made from commands or brought into existence by custom trades. 

Singleplayer custom trades have no issues reflected here.

In a test, in order of increasing frequency of the trades not registering to become successful, the following scenarios would have the correct input, but the input would "rubberband" back into the player's inventory: 

1. A single non-custom item for a custom item.
2. A couple of non-custom items for a custom item.
3. A single custom item for another custom item.
4. A single non-custom item and a single custom item for another custom item.
5. A couple of custom items for another custom item.

If the output was non-custom, the frequency of trades not registering was significantly lesser, but still present nonetheless.

This was tested before on older versions as well and the issue was present as well, but it cannot be confirmed until how far back it was present.