Zone names are required to be fully qualified with a final dot. However server names, when added to zones as NS records, are appended with a dot. Thus if they were entered with a final dot, we now have two dots, like so: ns1.example.com.. which causes an error when loading the zone.
In other DNS management interfaces, zone names are assumed to be FQDN and a final dot is not required. Sometimes if provided it is removed, for consistency.
With server names, I suppose we can avoid adding a dot when creating an NS record, if the server name already ends with a dot.