I highlighted one of my eBooks purchased from the Apple Books store on my professional Macbook, then waited and closed the app. Restart the computer, open the application and close the application. Now the highlights should have been synchronized. My iCloud is only half used (50%).
But when I opened this book on another Mac, none of the highlights were there. A Mac was running Catalina and a Mac was running Mojave.
Does this mean that I can highlight the whole book and that in the future it is likely that all of my highlights may simply disappear like steam?