From my experience any aoe that applies status effects can bug out any of the four legged beasts. Had it happen with firestorm, walking bomb, wrath of heaven (and/or spell purge), and once with elemental mines. It seems knockdowns can lag beasts into the walls where they probably drop off the map. It just happens so much with firestorm cause it's such a large woe and has a better chance of knocking beasts into walls.
So my advice is not to use any stuns/knockbacks on floors with beasts.
Likewise, i agree on knockdowns, but it doesn't event have to be in a wall.
Had one last night: it was a Darkstalker (little dinosaur thing?), standing in plain view, but no health bar, and immune to damage. We could still apply status effects to it (the text would flash up), so cold + Mighty Blow set off a combo on it, but it wouldn't die. We were moving it around the map by attacking it, so it wasn't stuck per se into terrain.
Threatening, Zone 4, Tevinter Ruins. Party was Katari (me), Necro, assassin, archer(?).
No red dot on the radar either BTW.