If the sync source is an instance of the PingDirectory server, then you should tell the PingDataSync server to record the record that is currently at the end of its changelog so that real-time synchronization can begin from there after it's started.
You can do this using the realtime-sync set-startpoint command. For example:
bin/realtime-sync set-startpoint \
--hostname sync.example.com \
--port 636 \
--useSSL \
--trustStorePath config/truststore \
--bindDN "cn=Directory Manager" \
--bindPasswordFile directory-manager-password.txt \
--pipe-name "LDAP Source to SCIMv2 Destination" \
--end-of-changelog \
--no-prompt