What you were expecting to happen, and what actually happened?
The thread title is self-explanatory. You hit it for 13 damage, it gains 13 attack.
What are the steps to make it happen again?
Use anything with split damage against anything with the Frenzy trait. i.e. Rowanne vs Berserker.
Do you have any screenshots or video you want to share with us so we can see the problem? Attach them to your post!