When you're placing a block ( for example a torch) with your offhand, you'll see the movement of your offhand which places the block.
But when you are in third person, the block will be placed by your main hand.
It will be the same if you change your main hand from right to left.
Thank you for your report.
However, your ticket has been resolved as a duplicate.
This issue has been linked to the original bugreport: MC-82965. You can find or add further information on that report.
If you haven't, please search before reporting any bugs, as it's likely that one exists already.