As stated in the title, when the player enters a villager trading window with a consumable in hand, the player will begin to and consume the consumable.

This bug occurs with food, potions, and special consumables (golden apples, rotten flesh, etc...). The bug appears to adhere to the general rules of saturation. For instance, if the player has full saturation, then food will not be consumed. However, consumables such as potions, golden apples, etc..., which can be consumed at any time, will still be consumed.

Likewise, the time for items to be consumed remains normal. This means, the bug is interruptible if the player leaves the trading window before consumption is complete.

I believe the issue lies in minecraft not registering a change in control input when entering a trading window and continues to read the input as "player holding right click".