When the Slide-Out animation is enabled and another display is adjacent in the direction a notification slides into, it'll briefly appear on that display before disappearing.
I'm not very surprised and not very concerned. I guess the fade-out/slide-out ratio can be tweaked, I'll take a look once I get to testing this in a multi-monitor setup. Feel free to submit a patch!
I've tested this now and I don't think it will look nice if the window fades out so quickly or slides out so slowly that it will not visually "reach" an adjacent display. So to be honest I don't think there's a solution to this. It's simply the expected behaviour with slide-out. If you want the notification to slide off-screen in a multi-display setup, you need to set your most-right monitor as primary (I know, may not be what you want) or you simply disable slide-out.
Would it be possible to mask it or make it transparent unless its on its designated monitor?
I don't think it's technically impossible, but I feel it's not worth the effort. I'd rather focus on other topics.