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I would thank to thank @whuber and @Chaconne for help. Especially @Chaconne, this derivation is what I wished to have for years.

The problem IS in the optimization part. If we set the random seed to 1, the default BFGS will not work. But if we change the algorithm and change the max iteration number it will work again.

As @Chaconne mentioned, the problem is squared loss for classification is non-convex and harder to optimize. To add on @Chaconne's math, I would like to present some visualizations on to logistic loss and squared loss.

We will change the demo data from mtcars, since the original toy example has $3$ coefficients including the intercept. We will use another toy data set generated from mlbench, in this data set, we set $2$ parameters, which is better for visualization.

Here is the demo

• The data is shown in the left figure: we have two classes in two colors. x,y are two features for the data. In addition, we use red line to represent the linear classifier from logistic loss, and the blue line represent the linear classifier from squared loss.

• The middle figure and right figure shows the contour for logistic loss (red) and squared loss (blue). x, y are two parameters we are fitting. The dot is the optimal point found by BFGS.

From the contour we can easily see how why optimizing squared loss is harder: as Chaconne mentioned, it is non-convex.

Here is one more view from persp3d.

Code

set.seed(0)
d=mlbench::mlbench.2dnormals(50,2,r=1)
x=d$x
y=ifelse(d$classes==1,1,0)

lg_loss <- function(w){
  p=plogis(x %*% w)
  L=-y*log(p)-(1-y)*log(1-p)
  return(sum(L))
}
sq_loss <- function(w){
  p=plogis(x %*% w)
  L=sum((y-p)^2)
  return(L)
}

w_grid_v=seq(-15,15,0.1)
w_grid=expand.grid(w_grid_v,w_grid_v)

opt1=optimx::optimx(c(1,1),fn=lg_loss ,method="BFGS")
z1=matrix(apply(w_grid,1,lg_loss),ncol=length(w_grid_v))

opt2=optimx::optimx(c(1,1),fn=sq_loss ,method="BFGS")
z2=matrix(apply(w_grid,1,sq_loss),ncol=length(w_grid_v))

par(mfrow=c(1,3))
plot(d,xlim=c(-3,3),ylim=c(-3,3))
abline(0,-opt1$p2/opt1$p1,col='darkred',lwd=2)
abline(0,-opt2$p2/opt2$p1,col='blue',lwd=2)
grid()
contour(w_grid_v,w_grid_v,z1,col='darkred',lwd=2, nlevels = 8)
points(opt1$p1,opt1$p2,col='darkred',pch=19)
grid()
contour(w_grid_v,w_grid_v,z2,col='blue',lwd=2, nlevels = 8)
points(opt2$p1,opt2$p2,col='blue',pch=19)
grid()


# library(rgl)
# persp3d(w_grid_v,w_grid_v,z1,col='darkred')