Server2016 - nslookup - Default Server

Solution 1:

The name nslookup prints as "default server" when it starts is pretty pointless (the name is not used for anything else than printing it there). It's just a case of nslookup having the OS resolver look up (reverse lookup) the configured resolver server IP address and presenting the resulting name to the user (if it could find one).
The actual IP address is printed on the next line (much more relevant, this is what is actually used by default).

As for your setup, with a reverse zone with multiple PTR records for the same IP mapping, that doesn't really make sense. I'd suggest that you have a single entry per IP, as is expected.

When there are multiple records in a reverse mapping, the choice of which one ends up being used by the client in any given situation is arbitrary.
If, as you say, nslookup ends up consistently using one over the other that happens based on arbitrary implementation choices along the way, there is no priority on the DNS level.

Sidenote: Other tools (such as dig) are generally favored over nslookup.