Enabling topos breaks PRACK in my simple (near default) proxy setup
(attached kamailio.conf). 

asterisk 13 (10.0.3.147) <-> kamailio (10.0.3.87) <-> asterisk 13 (10.0.3.154)

With the event route topos:msg-outgoing dropping all. The flow of
messages is as expected.

INVITE ->
<- Ringing
PRACK ->
<- 200 OK

With topos enabled, the PRACK doesn't get past route[WITHINGDLG] and is
answered with the 
        sl_send_reply("404","Not here");

INVITE ->
<- Ringing
PRACK ->
<- 404 Not here

These specific tests were done on kamailio 5.1.2, but with 5.1.3 I see
the exact same messages.