No BYE after receiving "491 Request Pending"

Hi all,
I have installed Asterisk 1.4, and I have a strange situation:

Inbound_Proxy — INVITE — Asterisk
Asterisk — INVITE — Outbound_Proxy
Outbound_Proxy — INVITE — Client
… 200 OK …
… ACK …
Inbound_PROXY — BYE — Asterisk
Asterisk — RE-INVITE — Outbound_Proxy
Outbound_Proxy — RE-INVITE — Client
CLIENT — 491 Request Pending — Outbound_Proxy
Outbound_Proxy — ACK — Client
Outbound_Proxy — 491 Request Pending — Asterisk
Asterisk — ACK — Outbound_Proxy

The problem is that the Bye is never send from Asterisk to the Outbound proxy and the dialog remains active on client’s side. Why Asterisk has this behavior? Is normal or is something wrong ?

I appreciate your time and answers, thank you!

Asterisk 1.4 is five years past end of life and six years past full support.