Image in spoiler.
The problem here is that I can't extend the brick wall. Trying from both ends, I get the option for either cornerpost, extend north, extend south but when I choose the option I want, the character moves toward it and then does nothing. I think the problem is either that the brazier is full of ammo, full of +0 purity ammo, or this is a meeting point of two brick walls that have separate starter corners. I have been able to build a brick wall next to full ammo and +0 purity ammo a few times before, but iirc that has always been the last segment running into a corner post.
The problem existed before I built the plank fence right up to the gap. That is not the issue.