Documented at https://akkoma.dev/AkkomaGang/akkoma/src/commit/develop/docs/docs/development/API/differences_in_mastoapi_responses.md#accounts

This is particularly important for viewing Akkoma profile pages directly (rather than by following a link on the frontend), for instance https://i.really.use.akkoma.btw.sylveon.social/users/agzgw9qukg7pxfcf will show "Sorry, there was an error loading this profile.", and this happens because pleroma.relationship.note that akkoma-fe expects is missing.