Problem is the curves don't hold. And either do too low or too high.
While manual tends to keep the setting .
It is general rule of thumb
I find there's only two reasons for fan curves not holding; one is if there's two softwares trying to compete over changing speeds.
And the other is if you have the fan speed update period set too high - which would be located just under the graph I linked above, if it weren't cropped out.