The Bug

You can't fuel a Minecart with Furnace with you off hand

To Reproduce

1. Place a furnace minecart.
2. Hold some fuel in your offhand.
3. Click on the furnace minecart.

Observed Result

Only a hand animation is played for the main hand, and that the minecart doesn't gets fueled.

Expected Result

You would be able to fuel the Minecart with Furnace with your off hand