Thanks, Teodor.
So manually adding the key to the script tag on the layout page does work, but trying through the Google Maps Properties does not (on saving, it removed the key I had manually added instead).
Not sure if this always happens, but I should also mention that saving the change on my content page (after adding the key via maps properties) does not trigger the need to resave the layout page… In this situation at least.
I tested that through the maps properties on the content page and it adds the key to the layout page.
Maybe something is wrong with your content page not properly connected to the layout page?
The API adds correctly into layout, the problem comes when after to modify the content page at trying to save it, for example adding a simple empty space into content page, then save, and the API key get remove from script in main layout. And this happend every time the content page needs to be modify, the API Key in the script in the main layout just dissapear.