No problem here at all. The half ring sits perfect. Did you connect the half ring properly to the top bearing cover?
I think this may be an issue with TanTan frames, with small differences from one frame to the next, maybe. My issue was never with setting the ring correctly. The TanTan ring would snap nicely into the TanTan bearing all the time. My problem was the stack height for the bearings + crown race + ring was never right.
I actually have two different headsets, the TanTan supplied one and an FSA ACR. The bearings are all 1.5 36 + 45 and all fit into the frame's 1.5" inch integrated cups snugly, without play, but they are all different heights. Between the 4 bearings, 2 rings, and 2 crown races, the stack height can be all over the place. Using any combination of pieces, I either had a stack height that locked up my steering against the head tube before tightening the headset, at the top cap or the forks, or even both, or just wouldn't allow tightening the headset at all.
Those spacers I got from my lbs were just the trick. They allowed me to dial in the stack height of the top bearing and ring. I just had to have a bottom bearing and crown race that fit well enough to not lock up the steering at the forks after tightening down the headset. All good now.
I think if you use the thicker TanTan bearing at the bottom, and the thinner bearing at the top, you can adjust the height at the top cap and still have enough space at the forks.
Hope thate makes sense.