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Yes I can confirm it's exactly that.

Basically Apple made a new "safe" API for screensavers (via .appex) but is only using it internally. 3rd party screensavers must still use the old unsafe API (that uses plugins, so unsafe code).

To bridge that gap, Apple made legacyScreenSaver, an .appex that loads the plugin code. It's a great concept in practice, but the implementation has been a mess since macOS 10.15 (Catalina...), breaking many things (multi monitor support was a big one, broken in many different ways over the years). Some Apple built in screensavers haven't been ported to the new API, so you may still have issues with legacyScreenSaver without that.

With macOS 14, Apple broke it again and indeed, legacyScreenSaver no longer tells the third party screensaver to quit! My rough guess is that this is linked to the way they implemented the transition with the new built in aerial videos they added that "slows" down on your desktop (on practice, it's not the screensaver that becomes the desktop, but 2 separate process/video players that gets synced up!).

Anyway, it's been a mess for third party developers for a while (I make one called Aerial). Our best workaround is to listen for a macOS event that tells us the screensaver will exit, and time our way to forcing an exit (literally, we call exit() which we shouldn't do, but that's the only workaround that works-ish). It works maybe 99% of the time. But sometimes legacyScreenSaver still craps out on its own.

Killing it manually will fix things (you will get a black screen + some CPU usage until you do, that can get bad depending on the screensaver) too.

I (and others) reported that bug multiple times but it's been 1 year+ and nothing. It's a mess.