I watched the video again and rep three looks just like the first two in every set, it must have been a function of me only getting 4 hours of interrupted sleep last night at work.
However, I don't know if its a function of the angle of the camera or what, but I see noticeable lumbar curvature (rather than flexion) in every set. This might be what your lumbar looks like when it's set and "flat", only your know your body.
Also, I still recommend lowering the weights and going with 8-10 sets of 2 reps. Your last two sets were very slow and non explosive. This is your first crack at RE Deads, go easy until you think you can start to really push the weight.