As I said you can reproduce this with stunnel "client = no" , telnet to "accept" port and tcpdump. As soon as you hit ^] and type "q" to close connection you will see "RST" coming from stunnel.
I understand that tcp/ip is not part of stunnel. Theres got to be some way to close() socket and have OS send RST.
Its very old BigIP version 3.3.1
Wow. Yep, that's old.
Version 9 has been out for more than a year.... I'd have no doubts that there are bugs in 3.3.1.