I too noticed that while I was writing the tutorial, but didn't know what was causing it at that time.
It happens because of the 'clean up' events, the condition checks if the number of neighboring wall times is more than or equal to 3, if it is less than 3 then convert the current tile to ground. It works good for all the tiles except the corner tiles, because no matter what, the number of neighboring wall tiles will never be greater than 3.
If we change the condition from 'less than or equal to 3' to 'less than 3', it fixes the corner tiles problem, but for some reason the 3rd tile is always blank or sometimes a ground tile. I have no idea why that happens.
I'll try to find out. Glad you liked the tutorial. :)