What I expected to happen was...:
When I hold items with the mending enchantment in both hands, and trade with villagers, both items should show the mending animation.

What actually happened was...:
Only the item in my main hand shows the animation when I get the experience. However, after closing out of the trading menu, both items show that they were mended.

Steps to Reproduce:
1. Hold a damaged item with the mending enchantment in each hand.
2. Do a lot of trading with villagers to get experience
3. Observe the animations while in the trading window. Only the main hand will show the mending animation.