Hello DamirH,
There is a setting within ‘Foliage’ mode that will allow your foliage instances to be accounted when the Nav Mesh is generated. Simply click on the static mesh actor while in foliage mode, scroll down to the ‘Instance Settings’ and set the ‘Custom Navigable Geometry’ to ‘Yes’ or ‘Even if Not Collidable’.
Now rebuild your scene to allow your Nav Mesh to be recalculated to account for your foliage instances that you have marked as custom navigable geometry.
Foliage Instance Settings
Foliage with Nav Mesh
Let me know if this was the appropriate solution, or if you are still experiencing issues.
Cheers,