It has happened in the past to me once for a trivial object, and occasionally when the game doesn't small save changes for whatever reason, but now it's happened for two of my objects at the same time and has removed everything in them. One object wasn't spawning things correctly and to simplify the problem I made a test level with only it and the player, but it still wasn't spawning some things with proper input. So I exited the editor and refreshed the game to reload everything, I do this a lot. But this time the behaviors in both objects were gone, I checked everything else and they seem fine. The first of these objects is what I was fixing when the problem occured, and the other was something that was being spawned correctly by the first. It's not a big deal and I will just rebuild it, but if this happened to other objects on a much larger game it would suck.