Showing the render without showing the entire AE UI with the modified properties of the Rotobrush layer revealed or explaining your workflow in detail does not help us help you.
The only thing that comes to mind is that you may not have frozen Rotobrush, or you may have a frame rate mismatch between the comp and the footage, or that the footage was trimmed, pre-composed, and some effect was applied to the pre-comp before running Rotobrush.
Have you checked the transparency grid with only the Rotobrush layer visible? Have you stepped through each frame in the timeline using Ctrl/Cmnd + right arrow? Are you using Rotobrush as a track matte for the text layer? Did you customize the render settings?
Post some details, and maybe we can help you.