Then the problem might be:

1. the layer used for the new collider is not set properly in the collision matrix
2. or the fact that one collider is a trigger
3. or you're moving the object by changing its position rather than controlling velocity using forces
4. or part of the object hierarchy is still falling at high speed and because the center of gravity is far below the ground it ends going through it

Without more description of what is going on, I can't think of another possibility.

Consider using the FixedUpdate method instead of Update for every changes that impact physics. The latter is framerate dependent, whereas the first one is executed at the same rate as the physics step.

Also note that adding/removing collider at runtime is a pretty expensive operation, so do not overuse it.

Agreed, there may be a better method to accomplish whatever it is you want to do... Anyway, your collider is being instantiated so that it's intersecting with the floor, therefore not colliding with it. Your method to edit the location gets it back on top, so it works.. or just instance the collider always above the top of the floor collider..