Since we are importing the same FBX file and under the same environment, it is not likely due to export or import settings.
In my layman’s mind, I expect that the number of polygons that can be handled on UEFN is limited. However, I am not sure if such technical reasons are the reason for low poly. Generally speaking, UE is likely to handle not loading meshes with a high polygon count.
You advice was perfect!
I can confirm that the building I created in Blender works perfectly in the UEFN world.
I will continue to work hard to contribute to the UEFN world. Thank you so much!