Mosaic effect tracking staying on screen after subject off screen

I'm trying to blur certain objects and text in a video using the mosaic effect. I can set this up with a mask that tracks the object through the footage with no issues whatsoever. 

The problem that Ive run into is that the mosaic effect seems to get "stuck" on the edge of the footage when the item goes off screen. This leaves part of the pixelation effect on the edge of the footage even though there is nothing needing that effect applied. 

I've tried removing the keystrokes thinking that would "delete" the effect past a specfiic timestamp, but that doesnt seem to work. 

What I'd like is a simple way to track an object from when it appears on screen to when it leaves the screen without needing to fiddle around with transparency or other settings. 

Is this possible?