This does appear to be the normal behaviour of the spotlight tool as it’s designed to be a spotlight for whilst in front of the classroom/presentation. You will also get the same effect happen on all other flipchart pages if you used the Revealer tool.
I would recommend either setting an object to have an action of the spotlight tool on the previous flipchart page ready for the flipchart page which you wish to use the spotlight tool on. Once you wish to finish with the spotlight tool, have another object to turn this off. Or alternatively, use the command which is Control + Shift + O whilst at the ActivBoard to turn this on or off.