I do have a reverse zone and the IPv6 records do show up in there. I see both "A" and "AAAA" records.
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This is not actually a problem as suggested above, It is just that server unable to get PTR record as the Reverse Lookup Zone is not configured on DNS server.
As your posted result, It looks like the you have only Reverse Lookup Zone for IPv4 and to correct this warning info you need to create the Reverse Lookup Zone for IPv6 as well