INET6_NTOA output is set as null to varchar(39) variable

The following code snippet was working for me in version 10.2.6. But I saw issues in version 10.2.9, where the variable "ip_full_addr" is returning null instead of expected value 2000::

 DECLARE ip_full_addr varchar(39) DEFAULT "";

 SELECT INET6_NTOA(UNHEX('20000000000000000000000000000000')) into ip_full_addr;

 SELECT ip_full_addr;