Warmog's Healing Passive Bug

One day, I was playing Dr. Mundo in a custom game and bought a Warmog's Armor. I noticed that even when I take damage, whether it is from structure, champion, minion or monster, Warmog's Armor would continue healing me despite the description noting that it will only heal when you have not taken damage for the last 5 seconds. I don't know if this is a real bug because I have yet to test it more than once.