Photoshop :: Liquify Filter Causing Semi-transparent Layers To Become Brightened

I'm having an issue with the Liquify Filter's final results. When using the filter it works fine, and if I use the filter on a solid layer its results given back to the original image are perfect also.
 
However, using the filter on a layer that has mostly transparency behind it (a layer with sketched line-art on it for example), the result given back to the original image is brightened significantly. The only work-around I have found so far is to merge a solid colour background to the transparent layer, and then the result works fine. However this is not desirable because future use of the image as line-art requires editing to get it back onto its own layer that can be destructive when the work contains toning.