Testing this out again internally here, I see that the oily harpoon sphere is applying the debuff with no issue. Some monsters are affected by it less than others depending on the strength of the monster. For example, on an Ogre, the effect lasts so short that it's barely noticeable. If you can get a video of this happening with a non-boss monster, that would be helpful to determine what the issue is. Thanks.

Fixed with the Trials Update as Sphere's system was replaced.