Tracking shape drifts off into space

I’m tracking a mostly stationary scene. I’m drawing a shape to track in a motionless background and I have Small Motion selected (and only Translation selected)

For some reason, the shape is drifting off the screen when I track forwards – even though there’s nothing moving in the shape I’m tracking.

Any idea what’s going on? There is a bit of grain (I have smoothing level set to 1).

Hmm, unless there’s like super8 levels of grain on it, the smoothing probably isn’t necessary. Can you try tracking a bigger shape? That might fix your issue. What’s the final goal of the shot?