AutoSync mode in Library means that metadata changes apply to all the selected images, and is normally enabled by clicking a switch in the right panel (F8). But what if that switch isn’t visible?
It took me a while to figure this out, but the switch is only visible when you are in Loupe view (E) and when multiple items are selected. So when you’re in Grid view, you get the “Sync Metadata” button. But when you’re in Loupe view, that button changes to “Sync” and it is inactive if only one image is selected. If multiple images are selected (via the Filmstrip) that Sync button has a little switch that lets you go into AutoSync mode.