Tricky. The spell says it targets the second last enemy, so when there’s only one enemy left it should obviously fail. Can’t hit what isn’t there. However, there’s been a similar bug report about this at the beginning of the year, in which @OminousGMan claimed this was identified as bug that would be corrected.
Given the lack of any further communication and seeing that the spell still behaves the same, I guess it’s pretty safe to assume nothing will be changed, no matter if this is a bug or not.