make sure you’re importing them into the correct file version of your latest backup that you’re actually using in Unity. (sounds dumb, but I do dumb stuff, and it’s the easiest fix)
delete your old Prefab and make a new one from your new .fbx file. Or, rather, make a new prefab (so you can copy and paste the old info that you need from your old prefeb) then delete your old prefab, or keep it, but make a brand new prefab from your new .fbx import. Sometimes, for some reason, a new .fbx might not always automatically update all the old info.