Master Yi's Highlander not working properly [GAMEPLAY]

I have been playing Master Yi more frequently, and I've noticed a bug to do with his ultimate, highlander. I've gotten multiple assists/kills in the duration of his ult over multiple games, but it does not seem to give him the additional seven seconds of highlander, as the tool-tip would suggest.

Step-by-step how to recreate:

1. Play Master Yi
2. Get to level 6
3. activate highlander and get a kill/assist
4. highlander doesn't extend.