When you have a particle system using emitters whose animation is on loop, every-time the animation loops there are particles (artifacts) showing up in the wrong places.
In the case of the attached sample program (BugExample.zip), the artifact can be seen in the middle portion of the screen. (See BugScreenShot.png)
Animation used is PathAnimation, with the loops property set to:
The ParticleSystem (LoadParticleQML.qml) in brief is:
main.qml file uses Loaders to load the above QML:
Also, this other Qt bug can also be observed using the attached sample program.