"is on screen" condition do not work properly with objects in a rotated layer. The visible or detection area for "is on screen" is rotated too. I have attached a cap file. There, moving the box draging or with arrow keys, changes the opacity with "is on screen" condition. Note that it changes in the corner (top-left, top right, bottom left, bottom right), because the screen is rotated too for the object.
"is on-screen" with rotated layers