Windows DNS NS record TTL

When editing a zone's NS records the TTL fields is always shown empty.

Examining the zone DNS file shows that the NS records may already have a value but it is not getting displayed.

I have noticed that when I enter a TTL into the TTL control on the Edit window for the NS record it updates the zone file sometimes which makes me think something is broken.

Does anyone have any knowledge of what may be happening here?

Edit 1

To clarify; why is the TTL for the NS records not populated in the control shown in the screen shot? It does not populate:

• if the TTL is different to the SOA default minimum TTL.
• if it is explicitly set to a TTL different to the SOA default.
• at all.