Interpolated handles not smooth

I’m seeing recently that extreme handles in interpolation is often off by 1 unit from the on-curve points. It’s not kinks, but seemingly something wrong with interpolation maths.

Attached is the test file. Reinterpolate the second master, and observe four extreme handles broken off by a unit.

Interpolation not smooth.glyphs (2.2 KB)

Thanks for the file. I fixed it.