[{quoted}](name=Teh Song,realm=NA,application-id=RaE1aOE7,discussion-id=xj5TdZZn,comment-id=0000,timestamp=2019-07-21T15:04:59.319+0000)
...they don't.
Healing HP+ is put against DoT hp-, both are happening. If one of those numbers is bigger than the other you aren't going to see them both happening, it will just look like the one is.
They do actually, google any TFT item sheet and read the item description there, or google something like "lol tft (item name)".
You can stack 3 warmogs on a guy and he wont regen squat if affected, heck they even take damage.