An easy fix here will be to use following tweak
iface wlan0 inet dhcp
<blah blah>
iface wlan0 inet6 dhcp
accept_ra 1
autoconf 1
pre-down wpa_cli -i$IFACE reconnect 2>/dev/null
this way it will try pre-down before starting down action. Pre-down eventually fails since wpa is already dead.
An easy fix here will be to use following tweak
iface wlan0 inet dhcp
<blah blah>
iface wlan0 inet6 dhcp
accept_ra 1
autoconf 1
pre-down wpa_cli -i$IFACE reconnect 2>/dev/null
this way it will try pre-down before starting down action. Pre-down eventually fails since wpa is already dead.