IPv6 RA trouble when upgrade 4.17 -> 5.9

I bumped one of our routers (RB1100) to 5.9 from 4.17 yesterday and now we’re having a bit of trouble with IPv6 router advertisements.

It seems the devices are getting an RA and the prefix is configured ok, everyone has a public IPv6 address. But the default (::/0) route doesn’t get installed in the routing tables so I’m wondering if the RA from routeros has been changed?

If I disable and enable the IPv6 address (under /ipv6 address), a new RA is sent out as the address is enabled and the default route is installed ok just like before the upgrade. So this “first RA” works. But if I now turn off Wifi in my device and then turn it back on, the default route doesn’t reappear. It worked under 4.17 …

Most of my config is defaults with just the prefix configured

/ipv6 address
add address=xxxx:xxxx:xxxx:f00d::1/64 advertise=yes comment="Wifi IPv6" \
    disabled=no eui-64=no interface=ether06-wlan

/ipv6 nd
add advertise-dns=no advertise-mac-address=yes disabled=no hop-limit=\
    unspecified interface=all managed-address-configuration=no mtu=\
    unspecified other-configuration=no ra-delay=3s ra-interval=3m20s-10m \
    ra-lifetime=30m reachable-time=unspecified retransmit-interval=\
    unspecified
/ipv6 nd prefix default
set autonomous=yes preferred-lifetime=1w valid-lifetime=4w2d

Any ideas how I should debug this further or have I missed some global configuration parameters for RAs in the 4->5 transition?