Some DNS records are in uppercase on slave servers. Even if those records are deleted and created anew in lowercase, they are still propagated in uppercase.
There are some records with uppercase tld in the DNS zone of the records in question. If that case PowerDNS treats all other records in the zone with the same tld as uppercase. For instance, if there is a record like
SOMENAME.L.GOOGLE.COM in the zone, all other
.com records like
example.com in that zone will propagate in uppercase, even if they are added in lowercase.
Please make sure that the zone in question does not contain uppercase records.