I expected to combine my stack of recently harvested beetroot seeds with an existing stack in my barrel by shift-right-clicking it in my survival world...

But what really happened was that my game closed automatically, my beetroot seeds were not placed there, but the game was saved before closing, making it so I came back into the world right before I opened the barrel.

1. Place down a barrel
2. Place some beetroot seeds (55) into it
3. Shift-Right-Click on the beetroots in your inventory (13)
4. Notice that the game closes automatically
5. Profit!