This still seems to be bugged for me. With only a single 1/1 enemy on the field, I used the bite ability. This killed the enemy but still reduced the skill power to 0. If the description of the skill says that it gets stronger after every use, having it get weaker for any reason seems strange.
Edit: I intentionally tried killing two enemies at the same time to see if that helped, but it still reduced the skill power to 0.