Cavitation is a good point, there are video of similar situation (including fenestron) often it looks like the flow of air already present is not letting water fully attack the blades, at least for a partial hit.
What comes out the event is a lot of mist; that would explain the absence of impact on the stability of the helicopter as the blade are turning (for that short time) in much less dense mix of water and air.
Lastly the aircraft structure hanging below is much lighter that water and would want to bounce back as a float would.
that does not count for the video bellow where the Fenestron got it all
https://www.youtube.com/watch?v=-wBgviMXKDE