DCP mutation message key has extra character for collection aware client

For documents in collections (and client using collectionAware), the keys returned from handling a DcpMutationMessage is including an extra character at the start of the key, eg. 

MutationMessage [key: "YSimpleKeyREP11781REP11781REP11781", vbid: 11, cas: 1587376336365617152, bySeqno: 415, revSeqno: 16, flags: 33554432, expiry: 0, lockTime: 0, clength: 180]
MutationMessage [key: "1SimpleKeyREP12441REP12441REP12441", vbid: 11, cas: 1587376339434536960, bySeqno: 420, revSeqno: 16, flags: 33554432, expiry: 0, lockTime: 0, clength: 59]
MutationMessage[key: "ASimpleKeyREP13057REP13057REP13057", vbid: 11, cas: 1587376341878374400, bySeqno: 425, revSeqno: 16, flags: 33554432, expiry: 0, lockTime: 0, clength: 46]

Keys are of form SimpleKeyREP...

The issue is not present when using the onMutation(Mutation mutation) from within DatabaseChangeListener, or when not using collections.

(Perhaps dataEventHandler is being deprecated for collections aware clients in favour of the DatabaseChangeListener ?)