Hi anan204
Thanks for helping me (again).
1) Yesterday I actually tested with Protostar and it worked just fine without this duality.
2) Unfortunately I do not know any other 3rd party extension that can be added into another area, using a {code} inside the custom html module. But the same happens when articles are published using the custom html module method. Please try selecting menu item "Article 1 (GRID6)" to see the issue repeated.
I think the said "page class suffix" could be the solution. Would you like to help me with the specific code that hides the green "MAIN" area shown on the screenshot above?