Reproduced again with your files. My project is located at C:\UE4. I renamed the noUV fbx to PlaneTest.fbx, copied it to my desktop, imported it, and applied the material. Then, I renamed the plane with UVs (7_4fbx_plane.fbx) to PlaneTest.fbx and copied it to my desktop, overwriting the original. I then right-clicked PlaneTest in the content browser and clicked Reimport. The UVs did not change.
I also tried overwriting PlaneTest.fbx with a model of a room. It remained a plane. I have attached the relevant log lines. Note that the number of “…/” in the asset’s path is just enough to backtrack to C:\ from “C:\Users\UserName\Documents\Unreal Projects\ProjectName\Content” but twice as many as is necessary from the actual working directory at “C:\UE4\ProjectName\Content.” The asset’s source directory is also set to “…/…/…/…/…/…/Users/Heaven/Desktop/PlaneTest.fbx,” which is also too many “…/”