If a hopper isn't connected to any container, and there are 5 stacks of items on top of it, they will all fill the 5 slots almost immediately.
On the other hand, if the hopper is connected to a container (such as a chest or shulker box) the items will only fill the first slot of the hopper: as one item from the hopper goes into the connected container, the stack will re-fill, but the 4 other stacks of items will be on top of the hopper, unable to be sucked in.
This is clearly unintended behaviour and it should be noticed that in the Java version all 5 stacks would be immediately sucked in regardless of whether the hopper is connected to a container.