As a lvl 71 Barbarian Playing online in ladder; when I cast howl on monsters in Act’s 3-5 as well as in lvl 85 areas (like the pit) it does not proc on normal mobs like it should. It seems to work correctly if you are a equal or lower lvl area.
Yes, that is by design. Your character level and Howl skill level summed need to be 1 higher than the monster’s level. See here:
https://www.theamazonbasin.com/wiki/index.php/Howl
Only causes normal and minion monsters to flee (with a few exceptions) for specified distance and duration if mlvl < clvl+slvl+1.
Now you know how Howl works, I encourage you to invest some points in it. I invested 10 points early on and levelled without rushing without any hiccups. It’s nice that Howl synergizes Berserk. For 1 point in Taunt, you’ll have 2 awesome crowd control skills.