@eskimo We are also getting reports of failures in first-party Apple screensavers using 13.4 on multi-monitor systems - in particular the Shifting Tiles screensaver has random blank screens when starting on 2 or 3 monitor systems.
Shifting Tiles: when using the "Options / Source: Colors" (which doesn't use photos, just blocks of color) it seems to work fine on 2 or 3 monitor systems.
However, if I choose "Options / Choose Folder" or "Options / Photos Library" then I can see some sort of malfunction:
With 2 monitors: after the screensaver starts, I get 2 blank screens for about 20 seconds. After 20+ seconds, then both screens start working.
With 3 monitors: after the screensaver starts, I get 5 seconds of blank screen, then monitor 1 starts, then another 5 seconds, monitor 2 starts, then another 5 seconds and monitor 3 starts. With repeated testing, the pattern is somewhat random: sometimes 1 and 2 start right away, followed by 3 about 5 seconds later.