When using a proof chain, secondary signers get to know who previous signers are. Which would reveal the same issuer based correlation as highlighted in issue 2 of the VC data model review in https://github.com/w3cping/privacy-request/issues/121#issuecomment-1638908803. This is not an issue with proof sets because each proof can be independently generated and combined during the verifiable presentation. For this reason, a privacy considerations section should be added to highlight this trust of data being shared between issuers when proof chains are used.
From the PING's review (https://github.com/w3cping/privacy-request/issues/120):
/cc @kdenhartog