I'm experiencing this internal DNS problem with Kamailio, can't seem to figure out what's causing it. Maybe someone can help.

Essentially, from time to time (one or two times a day) this situation happens, starting with logs:

linux kamailio[773]: ERROR: uac [uac_reg.c:1163]: uac_reg_send(): failed to send request for [some-uuid]
linux kamailio[773]: ERROR: <core> [core/resolve.c:1730]: sip_hostport2su(): could not resolve hostname: "my.domain.net"
linux kamailio[773]: ERROR: tm [ut.h:309]: uri2dst2(): failed to resolve "my.domain.net"
linux kamailio[773]: ERROR: tm [uac.c:480]: t_uac_prepare(): no socket found

# Do a Kamailio internal DNS lookup, no results found???
root@linux:~# kamcmd dns.lookup a my.domain.net
error: 400 - Not found

# Do a system DNS lookup. All good.
root@linux:~# dig my.domain.net
;; QUESTION SECTION:
;my.domain.net. IN A
;; ANSWER SECTION:
my.domain.net. 18000 IN A 192.168.0.1

# Delete internal Kamailio DNS record
root@linux:~# kamcmd dns.delete_a my.domain.net

# Lookup again right away. It's there, problem fixed. Why??
root@linux:~# kamcmd dns.lookup a my.domain.net
{
  name: my.domain.net
  type: A
  size_bytes: 120
  reference_counter: 2
  permanent: no
  expires: 7674
  last_used: 0
  negative_entry: no
  records: {
      {
         rr_idx: 0
         rr_ip: 192.168.0.1
         rr_permanent: no
         rr_expires: 7674
      }
   }
}