Reproduction steps:
1. Open the project;
2. Enter the Play Mode;
3. Change the object 'Cube' Y position a few times that you could observe the collision;
4. Turn on function 'Inflate Mesh' in Mesh Collider and set the Skin Width to 0;
5. Repeat the third step;

Actual result: Collision of the objects with Mesh Collider looks weird. The object stands on one edge or comes in other object or moves even if you stop changing Y position.
Expected result: Collision with Mesh Collider should look like the collision with turned on function 'Inflate Mesh'.

Workaround: When you turn on 'Inflate Mesh' in Mesh Collider, and set the Skin Width to 0 the collision looks correct.

Reproduced on: 5.5.0b10, 5.6.0a2.
Function 'Inflate Mesh' was published in 5.5.0b10, so there isn't a regression.