The green focusing square doesn't stay green very long. I read that you never take a picture with it still mag on 10, than you get the mag back to normal, by then the green square is gone and it is back to white, was that wrong? If that was wrong then no I did not shoot with the focusing on green. But it did look sharp on the lcd. I took a photo of my wife in a chair so nothing moved to change focusing plane and I had it on a tripod. I sent the lens in again but if it isn't fixed this time it is going on ebay. I did not send the camera in because it is not used on the same camera all the time. 2 weeks ago it was tested on a 50D and a 7D by another photographer with the very same results, back focusing with no recognizable face in the basketball team. Trouble is I'm still making payments on the thing.
You can shoot with the mag on 10, the camera doesn't care. Sounds to me like you are inadvertently resetting focus, and then getting focus on something else in the (much) larger green frame at 1x view.