CAS diff() giving incorrect results

[parisse]

f(x=...) will work if f is an expression. It will not work with e.g. (x^3)'' because the parser will not detect that it's an expression (and it will do like if it was implicit multiplication because x appears in both).
If you work with derivatives, it's a good idea to save them in variables. First f:=x^3'', then f(x=4)