Plot derivative of an ABS( function ) fails (RESOLVED)

[StephanP]

While testing the HP Prime I ran into the following odd behaviour.

Symb
F1(X)=|X^2 -10X +21| (between absolute brackets)
F2(X)=dF1(X)/dX=X (the derivative of F1)

Plot
F1 plots fine, bouncing nicely at x=3 and x=7
F2 does not plot well. It gives three horizontal line segments.
Y=1 for x < 3
Y=-1 for 3 < x < 7
Y=1 for x > 7

   

If I use the function Slope in the plot windows, then correct slope values are given for F1.
For F2, slope values are "NaN".

When I remove the absolute brackets from F1, F2 plots perfectly.