There is a problem with the SymPy trigsimp function when it is used to simplify an expression involving sums of products of trigonometric functions

Note that:

Qrotated.equals(a * xrot2 + 2bxrot*yrot + c * yrot2)
False

Qrotated = Qoriginal.subs({x: xrot, y: yrot})

SymPy first substitues x with xrot, some evaluation happens, then it substitutes y with yrot and some other evaluation happens.

Qrotated = Qoriginal.subs({x: xrot, y: yrot}, simultaneous=True)

According to the docs, help(Qoriginal.sub):

If the keyword simultaneous is True, the subexpressions will not be evaluated until all the substitutions have been made.

The docs also shows a couple of nice examples about this behavior.