loss is negative

I just run a demo program, in which I would love to perform on these data:
Actually, blue points are real data(x) while red points are z. The prior is N(mean of x, 0.01). Notice that I randomly set variance. Maybe 0.01 is larger than variance of x. Maybe smaller. I apply the default flows model in your ipynb. But loss varied from 800000->-30000->, and it is still decreasing. My question is how come negative loss would happen?
Plu, when I perform MAF/IAF,,,, loss would also be negative