Not really an expert on this, but Liquify will "stretch" the areas of colour. I don't know why the entire image is becoming reduced in opacity though as it appears to just be the right shoulder that is being moved. Are you Liquifying the entire image, or just a small area?
An alternative to Liquify would be to select the shoulder or area to be moved (using the Marquee tool), duplicate it on to a new layer (command/control + J) and use Free Transform with Warp (command + T, followed by a right click in the Transform box). Another alternative (there are lots!) is to just select the area that you want to liquify and see if the opacity change happens then.
Also, the screenshot is from the main window, yes? Not taken in the Liquify window with "Show Backdrop" ticked? Check the "Advanced Mode" settings if you mean that opacity is reduced when actually using Liquify.
Hope this helps!