Shulkers boxes drop some of their contents when broken by a piston.

I was trying to make a 5 way randomizer using 5 shulker boxes with different amounts of items to make different redstone pulses if read by a redstone comparator. When placed by a dispenser and broken by a piston, a small amount of the contents would empty out and messed up the randomizer system I built. Didn’t really know what to do so I just wanted to report it here. If nobody knows what’s going on, does anyone know a better 5 way randomizer design?