What is the bug?
The Explain API is not compatible with k-NN queries and assumes they have a score of 1. Well, I don't think there is a useful explanation to be given for k-NN queries, but when you have multiple k-NN queries or k-NN queries combined with text match queries, knowing the score of each k-NN part individually can be helpful.

How can one reproduce the bug?
Just make a k-NN query and set the explain flag to true.

What is the expected behavior?
Just have the score of the k-NN part correctly as the value field in the explanation.

Do you have any screenshots?