This should work but returns:

from 1
dig: couldn't get address for 'nsztm1.digi.ninja': failure
dig: couldn't get address for 'nsztm1.digi.ninja': failure
Zonetransfer.me on nsztm1 appears to be down
from 2
deadbeef.zonetransfer.me. 7201  IN      AAAA    dead:beaf::

Running all the commands from the command line they work fine

-----

#!/usr/bin/env bash

echo "from 1"
dig axfr @nsztm1.digi.ninja zonetransfer.me|grep dead:beaf
res=`dig axfr @nsztm1.digi.ninja zonetransfer.me|grep dead:beaf`

if [[ "$res" == "" ]]
then
    echo "Zonetransfer.me on nsztm1 appears to be down"
fi

echo "from 2"
res=`dig axfr @nsztm2.digi.ninja zonetransfer.me|grep dead:beaf`
dig axfr @nsztm2.digi.ninja zonetransfer.me|grep dead:beaf

if [[ "$res" == "" ]]
then
    echo "Zonetransfer.me on nsztm2 appears to be down"
fi