Problem with collision import from Blender

Hi, i began making a military base in blender and decided to import it to discover how collision worked. After many adventures and countless imports i did’nt manage to correctly import the collision mesh who is just the same mesh but with “UCX” before it’s name.

It seems ok for the “buildings” but there are infinite line going in almost every cardinal directions. I managed to make it work with the “use complex as simple” option but i would like to know why it did’nt and possibly how to fix it.

I used only a copy of the mesh because i plan to do only simple buildings shapes.

Here is the blender view:

And here is the base after import in unreal:

I unchecked “use one hull” during the import.

I would be very thanfull for any help to understand the mysteries of FBX import.