When there is no water in front of a dispenser with a bucket in it, the bucket itself is dispensed. This is very disappointing because if the chunks aren't loaded correctly, you could lose a perfectly good bucket.
What I thought would happen: The bucket would stay in the dispenser unless there is a water block in front of it.
What actually happened: I lost my bucket because it was dispensed into my mob grinder because the chunks were not loaded correctly.
What I think should happen: The empty bucket should stay inside a dispenser until there is a water block is in front of it. A bucket should only be dispensed as an item if placed in a dropper, not a dispenser.
This is probably intended. Because a dispenser can dispense item, so without a water (or lava) source block in front of it it will actually shoot a bucket.
You might want to try and keep the other chunk loaded so you can't lose your bucket because of this (try getting a hopper next to the dropper in the same chunk, directly facing the other chunk to keep it loaded). Otherwise you can try to move the dispenser back by one block so the source block and the dispenser (with the bucket) are in the same chunk so it can't dispense the bucket.