Some ISPs with /32 prefix do not take up the reverse delegation

I just notice some ISPs who have been allocated /32 prefixes from APNIC have not taken up the reverse delegation of their own address range.  They will face problem if the addresses are used to set up SMTP servers by their corporate customers.  One example is HGC who owns the prefix 2403:5000::/32.

[warren@dnssec ~]# nslookup
 > set type=ns
 >  0.0.0.5.3.0.4.2.ip6.arpa.
Server:         202.81.252.116
Address:        202.81.252.116#53

** server can't find 0.0.0.5.3.0.4.2.ip6.arpa.: NXDOMAIN

Here is a good example of CPCNet with 2403:2c00::/32

[warren@dnssec ~]# nslookup
 > set type=ns
 >  0.0.c.2.3.0.4.2.ip6.arpa.
 Server: 202.81.252.116
Address: 202.81.252.116#53

Non-authoritative answer:
0.0.c.2.3.0.4.2.ip6.arpa nameserver = ns1.hk.net.
0.0.c.2.3.0.4.2.ip6.arpa nameserver = ns2.hk.net.

Authoritative answers can be found from:
ns1.hk.net has AAAA address 2403:2c00:2::1