According to Epic Games.wall placement will work a bit differently in the upcoming Fortnite Patch 7.20. A fix was implemented to placing or building walls that become partially ground or are obstructed by the ground,causing players to build a second wall after the first on is built.
The next major Fortnite update is expected to go live following a period of downtime on January 15.
"The includes when a wall requires a connection to a building piece, or when the connecting piece is at least 70% underground. Under those circumstances, the required connecting piece will now build automatically and for free.In addition, the connecting piece will inherit team ownership rules(and the material type) of the desired piece".