What is says on the tin. All the buckets of fuel-related fluids the mod adds, that is, Crude Oil Bucket, Gasoline Bucket, Diesel Bucket, Kerosene Bucket, Naphtha Bucket, Heavy Oil Bucket, and Creosote Bucket, when thrown on the Item Drainer just tumble along its surface and, eventually, fall off.
Other buckets of liquids, such as Molten Steel Bucket, or Liquid Concrete Bucket, are drained as normal.

Temp fix is creating a "create emptying" recipe in a data pack, which will also allow you to then right click them into storage tanks when in creative as well