This seems to have been solved in 13.1 beta 2, thankfully. Can now move on to more useful work. But hope it won't break again, in subsequent updates.

A correction. Further observations. Plists that launch Privileged Helpers do get launched. Also, plists that launch periodic jobs appear to get launched. But plists that launch "Run At Load" executables are either not launched or there is some sort of a crash but I don't see any trace in the Console log.

I don't see anything related to any of the LaunchDaemons in the /Library/LaunchDaemons folder in the Console log. My feeling is that they may not even be launched, or no attempts have been made by the system to launch them on reboot, it seems.