Hi, recently posted here (cannot change category). https://answers.unrealengine.com/questions/535224/actors-briefly-walk-in-air.html#answer-535270
There is likely a bug with 14.4 PIE preview when you have the Landscape selected, then begin preview, causing randomly spawned or all actors to move for brief periods in the air.
Might be related to the visibility/rendering of the landscape later with PIE. Can upload a showcase video, showing this behavior. In my scenario, actors are spawned, and use Raycast Nav Mesh/AIPerception.