Root cause:

Previous connection SETUP log is record right after clicktcp_tcpconnect(), clicktcp_init_tcpconnect(). At this moment, APV just sends SYN packet to server side, TCP three-way handshake is not completed yet. When the SETUP log is record, the connection is not garanteed to be established. That’s why if there is no backend server existed, the log shows SETUP, DROP at the same time.
Solution:

Move the SETUP log to clicktcp_handler_spliced_synack() to make sure the connection is establshed.

Turn on log for NAT at APV
Set static NAT at APV
Send TCP request to APV’s VIP at client
Check the log buffer at APV if there is only DROP log.