To project and join walls I would recommend splitting the walls back from the corner a bit and delete the small sections of walls, then select the two walls, right-click on one of them and select '+Walls > Project and Join Walls'. This should fix the issue are having.
Here is a link to our wall tutorials. You can also access it via the orange question mark icon at the top of the Wall Tool Dialogue. All of our dialogues have this icon that will take you to the relevant tutorials.https://www.youtube.com/playlist?list=PL_je7dD7rzFujNxuvoPsFD9Ij-IG3rV0L