Comment # 2 on bug 1135647 from Bernhard Wiedemann

you could also try tcpdump -epni eth0
(or wlan0)

to see if IPv6 router solicitation messages come over the wire.

watching journalctl -f during ifdown $INTERFACE ; ifup $INTERFACE
could also give hints.

Indeed, dhcp4 mode still allows for IPv6 stateless autoconfiguration.