videobox causes random video distortions

I'm currently working at the project VOC2MIX which we just used at Chaos Communication Congress 36c3. So just a small thank you for GStreamer which we rely on. It mostly worked well!

We used a lecture composite where we crop the speakers camera picture so that it fits beside the slides.

After trying videocrop we had to switch to videobox because videocrop is not working together with GstController.

At the congress in some cases we got distorted videos in some cases and it seems that this is caused by the videobox element.

Videos were tripled in columns like we see the picture three times but horizontally shrinked or stretched picture.

If someone could give me a hint what could cause this behaviour I would be thankful. Otherwise I would see this as a bug.