Why is the voltage positive while the current is decreasing?

Simulation: -

Transient analysis: -

Maths: -

$$V = L\dfrac{di}{dt} = 0.4 \times \dfrac{-0.4}{0.01} = -16\text{ volts}$$

Conclusion: -

Why is the voltage positive while the current is decreasing?

It isn't; it's negative.

It's not the formula that changed but rather the definition of the inductor voltage in terms of direction.

A positive di/dt causes a positive voltage across the inductor and a negative di/dt causes a negative voltage across the inductor.

A similar question has been answered here The sign in the formula for the voltage across the inductor