I’m not sure if that’s a bug, it looks like it’s working fine. When you export the FBX it saves it all within one file but keeps all the objects as separate pieces. In Unity it looks weird because it has what looks like the same thumbnail. When you drag in each piece into the scene it shows up as a different type of model from what I see.
If there’s more to it than that then let me know, you could also attach the FBX file here and I can take a look at it.
