That's not the problem though - Logic *should* detect new plugins when you run it, without requiring a reboot.
But it seems an OS-related issue meant that the component that detects plugin changes fails to do this, so *any* application relying on the system to inform it of available system AU plugins should be affected the same (other, non-system plugin formats don't use the same mechanic, so those VST/VST2/AAX/etc plugins won't be affected the same way).
And it's the restarting of this component that is sometimes required (depending on what the newly installed plugin does) - rebooting does it, but do does just killing the audiounit registrar process, which is then restarted and detects the new plugins as expected.
So this is a MacOS thing, not specifically a Logic thing, and thus it can't be "fixed" by changing Logic.
Hmmmmm. I don't agree. When I click the "Rescan this Plugin" for any selected plugin within the Plugin Manager, it invokes some sort of routine (auvaltool, perhaps) that does an AU scan. So why couldn't Logic simply invoke the same routine for any "unscanned" components while its running? Its a simple matter to check the components folder for new items that haven't been qualified.