Is the well known textbook-formula for net-thrust oversimplified or do I have a misinterpretation?
You forgot to take pressure into account. This formula works assuming your nozzle is well adapted and the exit flow is at atmospheric pressure. In your case the velocity is increased but the pressure drops as stated in Bernoulli's equation.
$$P_0+ 0.5\rho V_0^2 = P_1+ 0.5\rho V_1^2$$
This means that the pressure force on your nozzle will counteract the thrust provided by the fluid acceleration.
See this explanation  your general thrust equation if the pressure are different is
$$\text{Thrust} = (Q * V)_e - (Q * V)_0 + (P_e - P_0) * A_e $$
Given $A_e$ is the exit aera of the nozzle.