When you place fluids (water/lava/powdered snow) in a cauldron in creative mode, an empty bucket is put in your hotbar. This does not occur in Minecraft 1.16.4, and is new to the snapshots. It is spammy to be given an empty bucket in creative mode, and seems to be a bug.

In these screenshots, I did not place the empty bucket in my hotbar. It just appeared there when I filled the cauldron.