When you place a block (a torch for example) with your off-hand, it moves your main-hand when looking in second or third person. It doesn't matter if your main-hand is left or right.
(See attached video)

What I expected to happen:
My off-hand moving.

What really happened:
My main-hand moving.

Steps to reproduce:
1. Put something in your main hand slot;
2. Put a block in your off-hand slot;
3. Press F5 once to enter third person mode or twice to enter second person mode;
4. Right-click to place the block in your off-hand slot.