Just for general info, the customer I'm working with has been in touch with hover.com and they (hover) may be using an affected version of PowerDNS, which may or may not be out of date.
[Interestingly, PowerDNS is perhaps part of the reason unboundtest.com was created PowerDNS: Can't find why CAA servfails - #47 by cpu ]
[Possibly related PowerDNS issue No SOA on ALIAS record types · Issue #4556 · PowerDNS/pdns · GitHub]