how to prefer temporary address? (raspbian / stretch)

Hello.

At the moment my raspy (stretch-based Raspbian) is getting a /128 static 
lease from my router out of the /48 I get from the he.net tunnel.
It also has a /64 address which seems stable across reboots but is not a 
slaac address.

Where does this /64 address come from? In the logs I see it mentioned by 
avahi-daemon and dhcpd but I cannot find a corresponding lease file.
More important, how do I get the sytem to generate and prefer a 
temporary address for outgoing connections?

Thanks,
Andrea.

P.S.: also posted here in the Raspberrypi forum:
https://www.raspberrypi.org/forums/viewtopic.php?f=29&t=249079