Yeah, the tooltip is bugged, but the good news is where you can get advantage from is mostly consistent. So once you have a visual of how much distance you need, you can get it consistently.
There are two situations:
1) The tooltip shows you won't get advantage, but the game is giving you advantage for high ground.
2) The tooltip show you will get disadvantage (from low ground), but you are in the zone to not get disadvantage.
