After depleting a stone pickaxe, any items placed from the offhand immediately disappear (torches and cobblestone tested). This persists in the slot where the pickaxe was depleted. If I switch my main hand slot to a different slot containing any item, or place an item in the slot where the pickaxe was at, I can again place torches and blocks from my offhand. I can also place torches and blocks from my main hand without issue. Any empty mainhand slots disallow the placing of blocks from my offhand even after rearranging my mainhand toolbar. While the items disappear, if I place something new into my offhand slot, the missing items reappear in my cursor as though they were there all along.