In function _grail_be_slice_state_update position_y is not updated.

static void
_grail_be_slice_state_update(struct _GeisSliceState *slice_state, UGSlice slice)
{

  if (slice_state->num_touches != grail_slice_get_num_touches(slice))
  {
    const UGTransform *C = grail_slice_get_cumulative_transform(slice);

    slice_state->angle = 0.0f;
    slice_state->position_x = grail_slice_get_original_center_x(slice) +
                              (*C)[0][2];
    slice_state->position_x = grail_slice_get_original_center_x(slice) +
                              (*C)[1][2];
    slice_state->radius = 1.0f;
    slice_state->num_touches = grail_slice_get_num_touches(slice);
  }
}

   slice_state->position_x = grail_slice_get_original_center_x(slice) +
                             (*C)[1][2];

I suspect this must be
   slice_state->position_y = grail_slice_get_original_center_y(slice) +
                             (*C)[1][2];