Ds-lite overrides IPv6 pppoe route


I have a Turris Omnia 3.8.4 running at a Wilhelm.Tel FTTH line. Since two weeks I have the situation that the IPv4 default route (with an official IP) get overridden by a dl-lite route after the wan6 interfaces is started.

Is there a way to tell the Omni not to setup a ds-lite when there is a working IPv4 connectivity?