When an item (in this case wheat) is held in the main hand and the crafting book is open, its shows up in the crafting book.

This however doesn't happen if the item is held in the offhand.

The issue is the same in the 4x4 crafting menu and if the "show only craftable" option is turned off.

Evidence is gathered in Lunar Client, but it's the same in Vanilla, (1.21.4)