When an equation in factored form has a term of an odd degree 3 or higher, and terms of other degree values, Desmos does not recognise that the derivative at a horizontal inflection point is 0.

Thanks for the report! The minimum is graphing correctly, which is easy since f’(x) is positive to the right and negative to the left thee, and we’re missing the other one, since it’s a second-order zero that never changes sign (f’(x) is negative everywhere near that line, so it’s hard to exactly find the one point where it is exactly zero. This is similar to the fact that we don’t plot point-discontinuities.