Ok. If you are seeing this from the future, and think you have a similar problem, and you are suspicious the issue might be due to a Viewport misbehavior or Pyqtgraph acting weird... It is very probably not the case.

After discarding that chance thanks to the comments, I was able to find the problem and it was entirely mine, a mathematical miscalculation on my program (I was positioning the parallel lines in regards to the reference one by only using the vertical distance between the points, and not the orthogonal distance [I am an idiot]).

The only useful coding related hint I can share for you to try to solve your problem in the future is: review whether you might using the same functions with different types of drawings (for example: the same distance translation function for entirely horizontal drawings and diagonal drawings).

Thanks to everyone that commented.

Cheers