Yeah, this feels like some sort of configuration issue on your end, because putting https://lemmy.one/comment/20361
in the box at https://lemmy.dupper.net/search should definitely work, and I can see it doesn't.
I don't think it's related to lemmy.ml exclusively if you can't get lemmy.one comments either... Without knowing what your configuration is it's hard to say.
It seems like if a user gets banned on their own instance, they show up on this list for everyone (I also see them on my ban list here), so unbanning them on your end probably does nothing.
Seems like just a bug, I think this list should only show local users who are banned, and instead it shows every ban it knows about across the fediverse.