Is either one a "multi user object" (that is, duplicate linked with Alt+D instead of simple duplicate with Shift+D)?
Do they have more than one material applied to them?
Do they have more than one UV map assigned to them?
Any of those can cause issues sometimes, particularly the first one in any version of Blender from 2.8 and newer. In 2.79 and older the FBX exporter supported older versions which worked fine for the multi-user objects, but since the exporter changed those objects tend to be broken in the KS editor; wrong materials applied or broken normals and the like.