In a UAC-Kamailio-UAS scenario, we've found a case where the ACK coming from uac is not relayed by our proxy to the uas. This is the log for the ACK message:
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: <core> [parser/msg_parser.c:624]: SIP Request:
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: <core> [parser/msg_parser.c:626]:  method:  <ACK>
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: <core> [parser/msg_parser.c:628]:  uri: <sip:59138553189@79.170.68.157>
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: <core> [parser/msg_parser.c:630]:  version: <SIP/2.0>
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: <core> [parser/parse_via.c:1286]: Found param type 235, <rport> = <n/a>; state=6
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: <core> [parser/parse_via.c:1286]: Found param type 232, <branch> = <z9hG4bKKc4gKXyemS9HD>; state=16
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: <core> [parser/parse_via.c:2561]: end of header reached, state=5
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: <core> [parser/msg_parser.c:511]: parse_headers: Via found, flags=2
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: <core> [parser/msg_parser.c:513]: parse_headers: this is the first via
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: <core> [receive.c:149]: After parse_msg...
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: <core> [receive.c:190]: preparing to run routing scripts...
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: sl [sl_funcs.c:396]: DEBUG : sl_filter_ACK: to late to be a local ACK!
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: maxfwd [mf_funcs.c:85]: value = 65
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: <core> [parser/parse_to.c:178]: DEBUG: add_param: tag=6ne1x6d6-CC-23
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: <core> [parser/parse_to.c:802]: end of header reached, state=29
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: <core> [parser/msg_parser.c:188]: DEBUG: get_hdr_field: <To> [52]; uri=[sip:59138553189@79.170.68.157]
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: <core> [parser/msg_parser.c:190]: DEBUG: to body [<sip:59138553189@79.170.68.157>]
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: <core> [parser/msg_parser.c:168]: get_hdr_field: cseq <CSeq>: <31362871> <ACK>
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: <core> [parser/msg_parser.c:202]: DEBUG: get_hdr_body : content_length=0
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: <core> [parser/msg_parser.c:104]: found end of header
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: <core> [parser/parse_to.c:178]: DEBUG: add_param: tag=0cS8trtaF196F
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: <core> [parser/parse_to.c:802]: end of header reached, state=29
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: sanity [mod_sanity.c:255]: sanity checks result: 1
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: siputils [checks.c:106]: totag found
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: ERROR: <script>: ESTAMOS EN WITHIN
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: rr [loose.c:108]: No Route headers found
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: DEBUG: rr [loose.c:829]: There is no Route HF
     Jul 27 10:04:59 theseus-test /usr/local/kamailio/sbin/kamailio[17358]: ERROR: <script>: NO LOOSE ROUTE 

However, in a pcap trace I clearly see the Route header:
This is the Record-Route in the 200 message
     Record-Route: <sip:79.170.68.157;lr;ftag=330cdf48-co1022-INS033>
And this is the Route header in the ACk:
     Route: <sip:79.170.68.157;lr;ftag=330cdf48-co1022-INS033>

I haven't been able to figure out what's going on. We have plenty of traffic working in that proxy. What am I missing?

Thanks in advance
David.