How to properly design mesh collider for level in Blender

I am editing the mesh collider of the 2013 Unity tutorial “Stealth”. I am using Blender.
I had to close a hole in the wall, and so I built another wall. You can see it marked with an “X” below

I used the New Face from Edges function in the Edit Mode (select opposite edges and press F).
For some reasons the original walls have an extra edge cutting the walls corner to corner, but I don’t know what it is for.

I apply the mesh collider to the level prefab in Unity and than I test, but for some reasons it doesn’t keep the character from walking out of the room, but then he keeps him from getting back in once he has crossed. So at least one side of the wall works? The other side has some weird element on one corner of where the gap used to be. I don’t know how it could make a difference.

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Here you can see my character’s little foot passing through in the image below (it was hard to capture the exact moment).

Did I built the wall in the wrong way? Is there some convention I am not aware of?

Solved. I just extruded the existing wall. For some reasons it worked better than building a new one.