It doesn't behave weird at all..
When the player is jumping it goes up, so when you change the gravity in the middle of jumping it goes up, which is down once the gravity has changed..
you could change this by adding an action after changing the gravity set the players platform vector y = 0..
This will make the player will stop and start falling..
If you don't want the player to stop setting the vectorY to -player.platform.vectorY will work..
player in your case being sprite2.