This is not really a bug, but more of a game balance issue.
I have noticed that the rate at which rain barrels gain water is about 8x slower than buckets. In the amount of time that it takes a bucket to be completely filled with 25 units of water from empty, a rain barrel only gains about 3-4 units of water. Given that a barrel should have a much wider opening than a bucket, barrels should gain water faster than a bucket.