I think it's a combination of two things. First of all, you definitely shouldn't use int() here, as this will stop the object about 20px early.
Another issue is Physics. It's not recommended to move Physics objects with non-physics actions. What happens is - Physics behavior is "resisting" and adjusts platform position, moving it a fraction of a pixel back. For example, if lerp set platform at Y=500, Physics moves it to Y=499.99995. Then you use int(self.y) and CurrentPosition value becomes 499. As a result, your platform can not move further.
If you remove int() and disable Physics, then your code will work correctly.
I suggest using LiteTween. You can achieve the same effect and it will stop at the exact position (unlike lerp, which gets infinitely close, but never actually stops). And then when the tween is finished, you can enable Physics.