I have that problem with the 8" ikelite dome on both the 10-22 and Tokina 10-17 lenses. Its a reflection of the lens front (on the 10-22 i could read CANON in mirror image on the thing. It was always worse on split shots OR very shallow water shots where the sun with *just* out of the frame. It ruined a fair number of shots as quite often it was too big to clone out. I tried various methods like a black marker pen over the lens from to hide the writing but nothing stopped it. The port itself is black so i assume its internal reflection through the housing itself. Short of duct tape over the entire housing front i don't know what to suggest.
On the 5" mini dome and 10-17 on the same setup i get no such issue.