Figured out what’s happening after consulting the source code™:
The path walker has a hardcoded distance threshold of 5 units for each target, so walkers will always cut corners 5 units in front of their goals (except for the last node, which the walker regards as reached 1 unit too early 
. If the playing field was larger, this would cause less of an issue, but since my units have a unit length of 0.25, this is a pretty grave violation.
I solved the problem by hacking around: each target point is extruded 5 units along the direction vector, except the last point which only needs to be extended by 1 unit.
Here’s my code that produces the correct, expected results
agent.node.setPos(self.node.getPos())
ai = agent.ai_behaviors
ai.pathFollow(1.0)
pts = []
lastpos = self.node.getPos()
for i,wp in enumerate(self.wp):
pos = Vec3(wp.node.getPos())
mpos = pos
if lastpos:
d = pos - lastpos
d.normalize()
if i < (len(self.wp)-1):
mpos = mpos + d*5
else:
mpos = mpos + d
lastpos = pos
pts.append(mpos)
for pos in reversed(pts):
ai.addToPath(pos)
ai.startFollow()