Frequent bug: while sorting by y-coordinate, the vertices are sorted, but the data coming with the vertices is not.
Gouraud shading, forgot to sort intensities

In my case I was calculating a barycentric coordinate of one vertex and assigning it to a different vertex by mistake.
The artifacts looked exactly the same as you presented, if it is the same problem then the description could be

I accidentally used the library like this:

$ svgexport a.svg a.png 500px
/Users/mark.campbell/Downloads/a.svg /Users/mark.campbell/Downloads/a.png png 100% 1x 0:0:301:79 301:79

This didn't do anything, and there's no --verbose option to check what the actual parsed arguments were when running it, so raising an error on the invalid value (500px) or having a verbose option would b

see=> Easy Scalable Text Rendering on the GPU (https://medium.com/@evanwallace/easy-scalable-text-rendering-on-the-gpu-c3f4d782c5ac)

BUT the original code (https://github.com/evanw/theta/blob/master/LICENSE.md) is GPL.

I can't look at the code => so I will implement it from his description.