Yes, I was changing the exposure settings the character blueprint.
Here is the camera settings in the player blueprint:
The leak does seem to persist regardless of the exposure level. Here is the mid level exposure that should match the editor view, but has the added leak:
Here is the camera set to a fixed high exposure with the light leak:
And here is the same scene with a low exposure fixed setting, with the leak still there.
I think I will place a Cine Camera Actor in the scene and see how that renders out an image. (Piloting it in the scene also had no leak, even though it had exactly the same setting for exposure as the player character camera.)