It’s a classic example of “fast bullets, thin walls” gamedev problem, except it your case it’s a feet Area2D instead of a bullet.
Physics is games are simulated in a discrete way. That means that a movement of a physics body is not continuous like in real life, it is simulated only in discrete points of space, 60 times per second. The state between these points is not evaluated. Strictly speaking a body just teleports short distances 60 times per second creating an illusion of movement.
What happens with “too fast bullets, too thin walls” is that at a frame 1 the bullet is right in front of a wall, but doesn’t touch it so no collision is reported. On the next frame the bullet travels a great distance because of its great velocity, so on frame 2 the bullet is already behind the wall. No collision is reported, because there was no frame where the wall and the bullet clearly intersect.
It boils down to: if a collision body moves with a velocity which length is greater than a thickness of colliders, it may go through said colliders without ever reporting a collision.
TL;DR: your feet area is too small for the velocity the player is moving when falling. And your floor is too thin. In cases like these people use raycasts with length set to the length of the velocity vector, not areas. Raycasts can report a collision at any point along the line of the raycast, not only on the very tip.