Shared Sessions misbehavior

[MonkeyTrainer12]

I have three different Joomla 4 sites which have started executing the "Shared Sessions" behavior, even though none of them are configured to do this. Two were Joomla 4 installations, one was an upgraded Joomla 3 site. There are no third-party plugins which are common to all three of these sites.

Logging in as a SuperAdmin on either side of the site makes a shared session active as both, logging out on either end deletes the session for both, and logging out on the front end occasionally throws a routing failure, possibly due to some sort of bad internal redirect.

I have a dozen other Joomla 4 sites which have never exhibited this behavior - though I know that I have never activated it on those sites, even as a test or experiment.

I can find no documentation as to how this process operates, but I assume my troublesome sites are failing to execute some test within a system user plugin during login/logout. Can anyone explain to me how Shared Sessions operates, which code or plugins make it happen, or even what configuration params or session flags make Joomla recognize that it is active?