The fix for your collision detection problems in Flixel.

CollisionFix.hx

/*
* From The Flixel Documentation
* The dimensions of the game world, used by the quad tree for collisions and overlap checks. 
* Use .set() instead of creating a new object!
* ------
* By updating the value of this element, we can fix our collision detection once again.
* Best to call this in your update function.
*/
FlxG.worldBounds.set();