RouterOS Version 6.36.4 (bugfix)
Board Name RB951Ui-2HnD

By RFC3315 dhcp-option clientid is required for Solicit, Request, Renew, Rebind, Confirm requests. But RouterOS DHCPv6 Client not send this option for Renew, Rebind (only this request tested, maybe for other requests too). Tested on ppp interfaces (pptp, l2tp, pppoe) with accel-ppp integrated DHCPv6 server.