Recalculating collisions after Physics2D.IgnoreLayerCollision()?

I have my "player" gameObject standing atop a platform. He press a combination of keys, I verify that he's standing on the right top of platform, and I run this code:

 Physics2D.IgnoreLayerCollision(playerLayer, plataformLayer);

The intention being that he'd drop down from the platform. Instead, nothing happen immediately. The player is still atop the platform and can move around - only if he gets out of it and tries to get back to it, only at that point will he fall down from it.

So I made a temporary ugly workaround:

 Physics2D.IgnoreLayerCollision(playerLayer, plataformLayer);
 rigidbody2D.AddForce(transform.up * jump / 1.5f);

This will force the player to jump, making Unity "recalculate" the collisions and the player will, as I want to, pass through the platform. The even weirder part is: if the jump isn't high enough (100 force at 1 mass with 0.5 gravity scale seems to be the minimum), the player will still land on the platform, even though Unity is supposed to be ignoring those collisions.

I also tried pushing the player downwards, both with force and direct velocity, but no luck - he still collides with the platform, he only stops colliding with it after being away from it once.

Also, my player Rigidbody2D Detection Mode is set to Continuous, and I tried setting the platform up in many different ways, with and without a Rigidbody2D.

Any ideas on how to make the player instantly fall down from the platform as soon as collisions start being ignored? Thanks in advance.