I have seen that aswell. They usually appear where other machines should be. I didn't take a screenshot, but I saw an image flanked by two ballistas during one siege.
This is an interesting and fun bug, but I also wonder if it could interfere with wanted siege gear if they spawn overlapping like that.On the topic of random siege ballistas, here is proof; this time two spawned on top of each other.
Here's my special report ?