Although i haven’t used YAST to configure DNS,
On general principles, the NS should be identified by its FQDN, not its reverse lookup entry.
Thinking about this further, I suppose that however incorrect it might be, if an appropriate mapping exists I believe that entering the reverse lookup entry instead might still work, this harkens back to my experience a long time ago when it was possible to list NS by IP address instead of by FQDN. Nowadays I haven’t seen identifying by IP addresses instead of FQDN used but that is probably more because it’s bad practice, not because it won’t work… So, because an IP address should also work it likely stands to reason that a reverse address entry <might> work.
The NS entry should be created even if it’s “the only NS in the LAN,” it declares where the zone lives (no assumptions needed).