I think you will see that this "ring reflection" may occur more often when you are shooting with the housing pointed towards the sun. If such is the case, then its likely what the ring reflection is the edge of the camcoder's body, (which surrounnds the lens,) that this camera surface is being illuminated by the sunlight and an image of this is being "seen" by the camera as a bright circular ring on the inside of your housing's port.
I suggest that you use either black marks-a-lot ink pen, or black tape, or some other physical method to black out the end of your camcorder.
I personally don't like the idea of using an ink marker to my little video cam, so one of the things that I may try soon on my camera to resolve this same issue is I plan to purchase and affix a dark black metal ring to the front of my camcorder in order to "mask" off the offending reflections. What I need is sort of like a screw on lens hood, but one that is very "short" and does not protrude, but also is very wide. I suspect for my particular video camera and housing that a simple COKIN A series Adapter Ring with the right thread size (39mm I think?) will fit on the end of my camera and this ring has a large outside diameter that should block out the reflecting edges of the camcorder body as well. One issue will be clearance for my flip down CC filter. Will just have to test it. And I will need to wipe off any white ink markings from the face of the cokin Adapter Ring (the white thread size ink markings) with acetone, and I may perhaps repaint it flat black using a fine spray modeling spray paint... but I suspect this will work well for my rig.