Hey Xukapy,
As Mieszko stated, the Nav Mesh generation is working as intended. If you’d like to make the desired changes and prevent the Nav Mesh from generating inside of the blocking volumes, you’re more than welcome to download the source and make these changes yourself.
Min Region Area is, in fact, working. The reason it’s not removing these smaller sections of Nav Mesh like what you’re seeing is because it’s only really meant to be used in isolated areas that are surrounded by areas that do not contain Nav Mesh. What you’re seeing is an area that lies on the edge of a tile, which means the Recast will assume that other tiles may contain Nav Mesh that can merge onto that tile, causing the size to meet the minimum region area requirements that you set.
That’s why I recommend using the Nav Modifiers in combination with Min Region Area changes to get your desired result. This is not a workaround, this is intended behavior. I recommend trying to keep your geometry in your level as smooth as possible to avoid any unwanted Nav Mesh generation as well on jagged or steep-inclined areas.
Let me know if you have any further questions regarding this, but we do not believe that this is a bug with the Unreal Engine at this time.
Have a great day!