Basically, I change the nature of how I show invisibility based on the demands of the narrative. Without the outlines in the first panel, the second panel becomes visually confusing. Did they fly up? Did they teleport? Did they cast Ghostform? Are they actual ghosts? With the outlines in place, there's significantly less doubt.

I had to invent the "Space Ghost effect" during the Darth V arc because I needed to show the pivotal moment when Vaarsuvius decided to go back and help O-Chul. The scene just didn't work without seeing V's face, but he needed to be invisible. At the time, I used the fiends' TV as an excuse for the change but now that I've used it once, I saw no reason not to use it again if it made the story clearer.

In short, utility trumps consistency.