In Mathematica 12.0, I execute the following code:
f(x_) = BesselI(0, x); f'(x)
that comes back
BesselI(1, x) as expected. But if I enter
f(x_) = BesselI(0, 1.0 x); f'(x)
0.5 (BesselI(1, 1. x) + BesselI(1, 1. System`Private`DerivativeX(1.)))
D(f(x),x) Returns the expected result. I tried to exit the kernel without changes. Is this a mistake or something is wrong with my installation?