Recognize the circular dependency
If a domain delegates to ns1.example.com while example.com itself depends on that delegation, a resolver may need the nameserver’s address before it can query the zone containing that address. The registrar publishes glue at the parent to break this loop.
Use glue only for in-domain nameserver hostnames as required by the delegation. Nameservers hosted outside the domain generally resolve through their own authoritative domain records and do not require glue in your parent zone.
Keep registrar and zone data aligned
When changing nameserver addresses, update the glue at the registrar and the corresponding A or AAAA records in the authoritative zone. A mismatch can send resolvers to the wrong server or leave some paths unreachable.
After a change, compare parent-side glue with authoritative answers and test from more than one recursive resolver. Remove stale glue only after confirming no active delegation still relies on it.