Every function evaluated at a specific value yields a constant and then you the the derivative of that constant which has to be zero - therefore it is not suitable as an IC.
In MC15 and below, to evaluate the derivative of a function you would have to write something like
but in a solve block with odesolve
does NOT work as IC.
You must use the prime symbol.
Its a pity that in real Mathcad we can't use the prime symbol outside of a solveblock with odesolve - a small benefit of Prime, where we can do so.