So originally I just wanted to change the material of the bridge lights, and I think i had deleted something, after that all of the materials became the same, like what you see in this picture, and the situation is still the same even when i re-import the bridge into the level,re-export it in blender and opening a new project in ue5, so what can i do to fix it? Thanks!
you probably have edited the original material that those material instances are based on.
a material instance is basically a child of an original master material. if you change something in the master material then all the children instances get those changes also.
this provides great flexibility but if you are not aware of this you can get in trouble.
try using the reference viewer to find the master/base material first