Hello @tuanphan
I've contacted SS help meanwhile and they told me it's a structural SS problem and they are dealing with it. Basically, when a page has not so much content it appears a blank space (if you have a coloured background, if it's white you don't even notice). I have solved the situation temporarily by adding another section, which gives the necessary height to the page, avoiding the blank space to appear.
Thanks anyway for your time!