It appears that macOS Ventura 13.3 beta (2) has introduced a severe bug with screensavers running on multiple monitors. This affects some of Apple’s own screensavers and appears to affect third-party screensavers such as iScreensaver.
Reported to Apple as FB12023530.
We will update this post as we learn more.
Ventura 13.3 beta 3 was released today, and unfortunately our tessts show that this bug is still present.
The RC version of 13.3 is out today (22E252) and the bug is still happening. Tnis is also being discussed here: https://github.com/JohnCoates/Aerial/issues/1286
Looks like general release 13.3.1 fixes the issue. Initial testing has it working same as pre-Ventura.
MacBookPro, 16in. - Ventura 13.3.1 - Apple/LG 4k as second monitor.
Update: 13.4 beta 1 seems to improve the situation (but only for 2 monitors). Other setups (such as 3 or more monitors, or with 2 monitors using rotated monitors) may still fail.
: we don’t think 13.3.1 actually changes anything. The bug is random, so please test a number of times (e.g. by launching the screensaver 10 to 20 times using a hot-corner). We see failure rates between 10% and 90% of the time.
It’s being reported that the first beta of macOS 13.4 fixes this issue, and our initial tests tend to confirm it.
For now: if you depend on running iScreensaver (or other third-party screensaver software) on 2 or more monitors, we are advising our customers to avoid macOS 13.3 for now.
Also, the bug can be demonstrated using some of the built-in Apple screensavers (Message screensaver is a good one to test).
Sadly, you are correct. 13.3.1 still yuck. Guess it was a fluke and wishful thinking. Trying not to do Beta so I’ll be patient 
We are pleased to announce that these issues are about 95% fixed in macOS Ventura 13.4, and appear to be 100% fixed in 13.5.
