Why is an incorrect average heart rate displayed after training?

This is probably because you did not stop the timer (stopwatch) in heart rate mode, and are no longer wearing the chest strap. In this case, the heart rate monitor is no longer receiving a heart rate from the chest strap, and because the timer is still running the watch is calculating with the heart rate value of 0. This means that the average heart rate will reduce further and further. Therefore, after training, stop the timer (stopwatch) in order to obtain a meaningful heart rate.