When using an item in your off-hand, the swing animation is played for your off-hand when in first-person perspective mode. However, the swing animation is always played for your main hand when in either third-person perspective mode.

Steps to reproduce:
1. Hold an item in each hand that's useable (for that hand). Example: Pickaxe in main hand, Sandstone in off hand.
2. In first-person perspective mode, use your secondary click (Use Item/Place Block)
3. Observe that your off hand swings.
4. Switch to either third-person perspective mode (the second or third mode).
5. In third-person perspective mode, use your secondary click (Use Item/Place Block)
6. Observe that your main hand swings, despite your off hand being used.