I think you guys broke something, all sensors seems to be broken when you reload a save. I've seen it with my saves and saves from other people, so it seems like a generalized problem. The only workaround I found is to change the sensor from above to below, and back to what it was.
What I mean by broken, is that they don't turn on when they should, they just stay inactive forever.
Not sure if it will happen if you create a fresh scene with sensors, save it and reload it. But it sure does it if you open a scene from before the last 2 updates, the updates posted today, not sure which one of the two.