Not getting Overkill achievement

Using a Diamond sword (8 hearts) with Sharpness III (+ 3.75 hearts) and using a Strength II potion (+260 % attack damage), my sword should be doing 30.55 hearts in damage, well above the limit for getting the Overkill achievement. Tried attacking chickens, sheeps and zombies, but no achievement. (Numbers based on current wiki numbers).

If based upon in-game numbers, it says that the Diamond sword with Sharpness III has an attack damage of 10, and still that Strength II gives +260%, which should add up to (at least) 26 attack damage.  If that translates to half the hearts in damage, it should still do 13 hearts of damage, well above the requirements for the achievements.