Hi ysungur,
According to SOL9812, LTM should send a RST after getting the SYN ACK. If you're not seeing this, you could open a case with F5 Support and ask them if the expected behavior has changed in v10.x or if this is a bug.
https://support.f5.com/kb/en-us/solutions/public/9000/800/sol9812.html
Certain BIG-IP monitors use a TCP RST packet to close the monitor connection quickly. For example, the tcp_half_open monitor performs a simple check on the pool member service by sending a TCP SYN packet to the service port. When the monitor receives the SYN-ACK packet from the pool member, the monitor considers the service to be up, and sends a TCP RST packet to the service instead of completing the three-way handshake. The TCP RST packet is typically sent on the server side of the connection, and the source IP address of the reset is the relevant self IP address of the VLAN.
Aaron