Federation shenanigans would be my guess. If you were on instance B and moderating instance A then instance B admins would have control of your account instead of instance A admins. So if for some reason instance B admins decide to ban that user then they are prevented from moderating for instance A.
there are bugs on moderating communities on other instances. like mode actions do not propagate to federated instances if done from a different instance.
but don't take my word for it, i don't have a source.
I've experienced issues like reports arrive late for external moderators, and post actions only work, and not comment and user actions.
https://github.com/LemmyNet/lemmy/issues/3781