The Hypotenuse Blueprint node currently returns a value of "-nan(ind)" if both values passed into the node are "0.0". This result is correct based on the code that the Hypotenuse node uses, but is not what is expected based on the Pythagorean Theorem. Based on the Pythagorean Theorem, the expected result would be "0.0" (ignoring that a "0.0" length should be impossible).
The top left corner of the viewport shows all three values of the Vector as "-nan(ind)".
The top left corner of the viewport shows all three values of the Vector as "0.0".
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-69468 in the post.