I just took a look at this. My monitor is 1920x1080 and I see it too.
The graphics resize about 1 second after you switch to full-screen. There is no information lost when these thin bars appear, it is resizing to an aspect ratio that doesn't fit the monitor exactly. Rather than show the art squashed or stretched in one direction only, it adds these thin bars at the edges.
I would have to dig into the code to find out exactly what aspect ratio is required by the game, but this seems like it is not a bug and probably "working as intended".