Please post a picture of your Polaroid or Fuji instant film holder, so we can see the darkslide area.
Occasionally you'll find some of these holders that actually do not work with the Graflex back. For some specific market or camera, they were made with the opening slightly adjusted to the middle. I had one like that and on my Speed Graphic it did not quite sit correctly in the camera and would leak light just like that.
The solution was getting another holder. I discovered the discrepancy after a lengthy discussion about these holders on the Large Format forum.
The difference in placement of the film area is very slight and it can be hard to tell without comparison, and impossible to know unless you are aware of the problem. There are no identifying marks AFAIK on the holders.
There may also be a tear in the Speed Graphic focal plane shutter if you used that, or light leak in the bellows. Have you tested / looked for that issue?
I highly, highly doubt there is a light leak in the lens, but one other thing - there aren't any open holes in the lens board right? Sometimes these lenses come with a solenoid that people remove, which leaves a couple of little screw holes in the board. They can be patched up with electrical tape. Normally though it creates a faint double-image, like a pinhole, not a flare like that. Ask me how I know...