Describe the bug and expected behavior
Current behavior:
No response when running a Bi-directional Reachability question with the TCP flag set to SYN.

Expected behavior:
If TCP flow occurs from server 3 to server 1, the flow is allowed because dev2's ACL is set to "established."
The expectation is that can get response data.

Consideration
I got the correct response with the TCP flags below.
・MatchTcpFlags.match_synack()
・MatchTcpFlags.match_ack()

I think that this problem, the TCP flag (ACK bit) of the return flow is not converted.
(it is thought that the above working pattern was that forward flow already set up a ACK flag, so the return flow was also working fine.)

Config

hostname dev2
!
interface GigabitEthernet0/0
 ip address 10.0.12.2 255.255.255.0
 ip access-group 100 in
!
interface GigabitEthernet0/1
 ip address 10.0.23.2 255.255.255.0
!
access-list 100 permit tcp any any established
access-list 100 permit tcp any any eq 22

{
  "hostname": "server1",
  "hostInterfaces": {
    "eth1": {
      "name": "eth1",
      "prefix": "10.0.12.1/24",
      "gateway": "10.0.12.2"
    }
  }
}

{
  "hostname": "server3",
  "hostInterfaces": {
    "eth1": {
      "name": "eth1",
      "prefix": "10.0.23.3/24",
      "gateway": "10.0.23.2"
    }
  }
}

Runnable example

bireachability = bfq.bidirectionalReachability(
                    pathConstraints=PathConstraints(startLocation="server3"),
                    headers=HeaderConstraints(srcIps="10.0.23.3", srcPorts="50001", 
                                              dstIps="10.0.12.1", dstPorts="23",
                                              tcpFlags=MatchTcpFlags.match_syn(), ipProtocols='TCP'),
                    returnFlowType='SUCCESS').answer().frame()
if bireachability.empty:
    print("no data")

Sample Config.zip