Some applications, like Discord or Mastodon, allow a user A to put private notes (only accessible to A) next to the profile of another user B. This data is very probably Personal Identifiable Information (PII) according to GDPR: it is linked to a specific profile of B, but also to the profile of A.
If user B wants to access to their data per GDPR, can / must you give user B the content of the notes entered by A about B?
It seems a kind of breach of privacy because before a data-access request, the information in the note was information only known to A. To me, it seems that A is responsible for the data entered, but it seems the operator of the service still has PII about B and should probably give the data to B if B asks about it.