This glitch is also present in RMXP:
The skill used in these screenshots is Radiate, a default RMXP skill that targets all enemies and uses an animation with its position set to "screen". In the left screenshot, it's being used against a single enemy. In the right one, it's being used against a group of five enemies. The left one looks alright. In the right one, the five redundant layers make it hard to see anything at all.
This apparently happens because setting the animation position to "screen" does nothing to prevent it from playing via the battlers. It merely changes the origin coordinates so that it plays at the center of the screen instead of at the center of the battler. Other than that, it works exactly as a "middle", "top" or "bottom" animation.
Unfortunately, I haven't found any fixes for the XP version of this glitch. If anyone knows of one, or is able to make one (YanFly's script might be useful in that regard as a reference of sorts, but being RGSS illiterate, I wouldn't know), I'll be very grateful.
Thanks for taking the time to read this request.
Nevermind, someone has helped me out with this already. Thanks anyway.