The prompt encoder's affine step (2*coords - 1, true range [-1, 1]) was quantised with scale=0.0045211, zero_point=52, which only reaches -0.2351. Every normalised coordinate below 0.3820 saturated to the same value, so the left and top 38.2% of the frame was a dead zone: taps there all decoded as a tap at 0.382.
v2 sets scale=2/255 and zero_point=128, giving [-1.0039, +0.9961]. The QuantizeLinear and its matching DequantizeLinear share the pair, so the QDQ stays self-consistent; only range and step change (step 0.0023 -> 0.0039 in normalised coords, ~2 px at 512).
Published alongside the original rather than replacing it, so existing clients pinned to sam2-split-w8a8.zip keep working.
