I say it again
Like Dani said balooning effect is normal on any aicraft.
Now have you ever had a vertical upgust on an airbus fly by wire.
The aircraft (if AP on) flies at 0.1 g. It damps so smoothly that you can gain several hundreds feet if you do not disconnect the AP.
The same happens when you extend the flaps especially at F 2 because you have such an amount of lift.
Check your FCOM FCTM guys