This has been seen with Soulus-1.12.2-5.0.0-f7f770c.jar in the modpack Volcano Block-1.0.39.
Maybe the game considers it a fuel, so adds burn time to the furnace, but being a bucket, expects it to not be the fuel itself but rather contain the fuel, so tries to empty it rather than consume it.
Maybe it works similar to #59
This has been seen with Soulus-1.12.2-5.0.0-f7f770c.jar in the modpack Volcano Block-1.0.39. Maybe the game considers it a fuel, so adds burn time to the furnace, but being a bucket, expects it to not be the fuel itself but rather contain the fuel, so tries to empty it rather than consume it. Maybe it works similar to #59