I thought I was going crazy! I'm having this same issue. I'm currently working on a mod that makes the special guests (officiant, ring bearer, etc...) all wear the same outfits by default and I noticed it wasn't working as it should. I then ran tests in vanilla and see that this is a bug.
Regardless, I've made a temporary fix (mod) if anyone wants it for their custom weddings. All guest will actually show up in formal wear. Let me know if it's something you'd like.
As for the funeral, I already made a mod on CurseForge called Better Funerals. It takes it a step further by actually making guests wear black to the funeral.
