The problem is that the game prefers to try and plant something on a block which cannot be planted on instead of eating from the offhand slot.

How to replicate:

1. Take any food item and place it in offhand.
2. Take any seed/plantable that is not edible and place it in main hand (e.g. wheat seeds).
3. Now stand in range of a non-farmland block and hold right click.

Result: Nothing happens, because the game tries to plant instead of eating.
Expected result: You start eating, because what else would you want to do? Planting is impossible on this block.