It’s hard to tell from your images, but I think this is simply due to you having a too low resolution for your graph.
Think of the resolution like a pixel image, if you have just the right resolution it might be able to clearly see the corridor, but if it is nudged just a little bit the pixels for the corridor might overlap with the ones for the wall and then it doesn’t work so well. Try for example doubling the resolution, I think you will get a much higher quality navmesh then.
On the other hand, it might be because your character radius is quite high. While the default character controller radius in unity is 0.5 meters, that is quite a lot higher than an actual human being and it might make it hard for a person of that size to squeeze through narrow door openings or corridors. Try lowering it a bit and see if that helps.