Issue can be easily seen with the Mussel test scene:

The indicated join point overlaps the triangle of the lower quad curve. That causes filling errors. (The reason there are two, and not one, artefact triangle in the picture is that the straight line actually consists of two line elements)

There is initial code to handle this behind QT_QUICKSHAPES_WIP_CONCAVE_JOINT. However it is marked as inefficient.

[Previously it also consistently caused a cascading splitting, so the problematic curve was always divided into 512 subcurves, and an error message was printed saying that the splitting limit is reached, but I can no longer reproduce that].