The current macOS version 13.3 broke something with the screen saver’s handling of external monitors, and the 13.4 beta may not have fixed everything.
First, what will get better. When using the macOS screensaver function with 13.3, it can happen that the external screen – for example the one connected to a laptop – remains desperately black. No need to shake the cable, the bug is software. We were able to check it on a Mac in 13.3 and see that it worked again on another which has already gone to 13.4 b1.
However, another problem has been noticed by several users equipped with two or three external screens. They complain about it especially here with Aerial software. The screensaver image orientation — when one has a screen rotated to portrait mode — isn’t always taken into account.
Or else, screens will remain black apart from the main one. There may also be a problem with the ratio of the displayed image, it does not occupy the entire surface. In one case, the person says they are still on 13.2, which means the bug was already hanging around.
The linked discussion brings together users of the Aerial screen saver but the author of iScreensaver found these issues as well. Apple screensavers don’t seem spared either.