I've found the best way to handle this kind of platform is with a state machine setup. You want to be able to prevent "tunnelling" in your physics, and this helps with that.
Basically you create two states "above" and "below". If the player is above, then he collides with the platform. If he's below, he doesn't. You use the line segment representing the top of the platform to handle the state changes.
1 | 2 | 3
| |
----===========----
| |
4 | 5 | 6
(Equal sign is the platform. Other lines show different areas)
Basically you make a state machine where 1, 2, 3 are "above" and 4, 5, 6 are "below". Then you allow state changes both ways for 1 <=> 4 and 3 <=> 6 but only one way for 2 and 5, 5 => 2.
Then, to handle the Drop Floor problem, all you do is swap the state to "below" and the character will stop colliding with the platform.
Hope that helps, even if you already found your solution
.