Possible to solve this kind of jonction problem??
Have you used the Wall Extend command on the “A” wall? If so try setting its height until the red line. The problem is that there is a limitation on the join between extended parts of walls.
Then if this doesn’t solve the issue select both walls and try changing the joint wall type from the Properties panel.
If you want you can send the model to email@example.com so we can take a look at this particular case.