Floor faces that lead to a corner are slightly raised when baked to a collisionpolygon2D
I've noticed something consistent with making corners in smartshape, it consistently generates a slightly raised angle. Not sure if this is a godot problem or not, but it can create problems for people making character controllers compatible with smartshape.
The first image is the corner as shown by smartshape's UI, and the second shows the corner as a raw collision polygon. If you look very closely on the second image, you can see there's an ever so slight deviation in elevation despite this being created on-grid. The ground angle that gets printed when walked on in-game is 0.0918511284681.
(The grid being offset in the second screenshot is because of godot's weirdness and doesn't have an effect on it.)
I created a rectangular shape with grid snapping enabled und couldn't reproduce the issue. Can you provide an MRP or reproduction steps?
Are you using Precise collision mode? If yes, do not use it unless you are absolutely sure you need it.
With that enabled I get non-integer position values, but they still line up equally.