By the way, I think I may have found that the event "(Invert) Is on-screen" doesn't work. If so, for now the "Else" command can be used to get around it. ("Is on-screen", "Else")
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As far as I've used it, "is on screen" worked as intended.
If you have a "non-working" example, please provide a .c3p reproducing the issue you are mentioning.
It it is validated as a bug, you will possibly have to fill up a report in C3's bug database for the devs to investigate it and fix it.