nsupdate on Solaris produces different failure text than expected

The nsupdate system test fails on Solaris 11.4 because nsupdate fails with "failure" where "not found" is expected:

I:nsupdate:ensure unresolvable server name is fatal in non-interactive mode (40)
couldn't get address for 'unresolvable..': failure
syntax error
I:nsupdate:failed
I:nsupdate:ensure unresolvable server name is not fatal in interactive mode (41)
couldn't get address for 'unresolvable..': failure
I:nsupdate:failed

n=`expr $n + 1`
ret=0
echo_i "ensure unresolvable server name is fatal in non-interactive mode ($n)"
$NSUPDATE <<END > nsupdate.out 2>&1 && ret=1
    server unresolvable..
END
cat nsupdate.out
grep "couldn't get address for 'unresolvable..': not found" nsupdate.out > /dev/null || ret=1
grep "syntax error" nsupdate.out > /dev/null || ret=1
[ $ret = 0 ] || { echo_i "failed"; status=1; }

n=`expr $n + 1`
ret=0
echo_i "ensure unresolvable server name is not fatal in interactive mode ($n)"
$NSUPDATE -i <<END > nsupdate.out 2>&1 || ret=1
    server unresolvable..
END
cat nsupdate.out
grep "couldn't get address for 'unresolvable..': not found" nsupdate.out > /dev/null || ret=1
[ $ret = 0 ] || { echo_i "failed"; status=1; }