QPainterPath produces a band when drawing a line between two points for a certain range of distances

Intro:
I am trying to draw a graph that shows points connected through a line. I am using QPainterPath to connect the points which are of type QPointF.

Problem:
I am seeing a band when drawing a line between two points. This only happens when distance between the two lines is in a certain range.
This band is an irrationally thick line that looks like a rectangle. This line shows somewhere in between the two points and not all the way from end-to-end.

Expected:
A normal smooth line between two points regardless of the distance between them.

Reproduce:
I have attached a small application that you can run and see the band. A README file is also attached to describe more about the application and the problem.