I've got cloudflare and tunnelbroker DDNS set up, and it all works fine, apart from the last update value, which seems to default to Unix time 0.
vyos@vyos:~$ show dns dynamic status ip address : xx.xx.28.90 host-name : *.domain.tld last update : 1970-01-01 00:00:00 update-status: good ip address : xx.xx.28.90 host-name : tunnelxxxxxx.tunnel.tservx.xxxx.ipv6.he.net last update : 1970-01-01 00:00:00 update-status: good