So I managed to save the blank prefix on the controller using @benoire's instructions, but our USG wasn't getting any ipv6 still. I was searching around today and found a workaround:
https://www.noodles.net.nz/2018/09/20/ipv6-workaround-for-unifi-usg-on-2-degrees-ufb/
ymmv, but it fixed it for us
