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.