Unbound not serving PTR records for private address space

Issue

We have unbound configured as a caching-only name server:

# grep forward-addr /etc/unbound/unbound.conf 
    forward-addr: 10.10.1.2@53
    forward-addr: 10.10.3.2@53

When we do a reverse-lookup for an IP address (a PTR record lookup) using the local unbound service, NXDOMAIN is returned:

# host 192.168.1.3 127.0.0.1
Using domain server:
Name: 127.0.0.1
Address: 127.0.0.1#53
Aliases: 

Host 3.1.168.192.in-addr.arpa. not found: 3(NXDOMAIN)

However querying the forwarded name server works correctly:

# host 192.168.1.5 10.10.1.2
Using domain server:
Name: 10.10.1.2
Address: 10.10.1.2#53
Aliases: 

5.1.168.192.in-addr.arpa domain name pointer host.example.com.