It looks like the rDNS delegation of the routed prefixes stopped working some time ago.
I did this a few years ago and I'm now trying to do so again on a new tunnel, but to no avail.
Either clicking on "delegate to dns.he.net" or setting any other namserver on tunnelbroker.net appears to work, but in fact the NS records are not being added.
Also have this issue, delegated networks to dns.he.net disappeared from dns.he.net panel, but NS records still working, just can't edit, add or remove existing.