Ohhhh this might be it actually! Thanks a bunch for sharing - this has been doing my head in ever since I got this monitor (Ultrawide Alienware if it matters).
This screenshot is from 30 seconds after this was posted. For context, the app I've got open is Kate, and below it is Firefox. The line is much longer now, but same width. The lines also never change in width, only in length.
Really hope they fix the broken notifications system. Currently you have to manually enable notifications and its not a very seamless process. Took me a good while to figure it out at least and wasn't a great experience.
you guessed it!