Double-check your actual step wedge "calibration" densities.
I don't think the jump is that dramatic, but it's interesting.
When I drew a smooth dotted line between points, the "outlier" on the 9,75 curve was the point for Step 17.
I see your calibrated value for Step 17 is 1.60. The "outlier" "fell into line" when I changed that value to 1.61. So double-check, maybe your calibrated wedge is 1.61