F5 LTM Sending ACK to a RST,ACK on its own with only routing+nat configured

There are some VS in front end network with pool members in the same network as the server, but the server with problems is disabled, the node is disabled all the pool members and the monitor instances are disabled. We must resolve this problem before it goes into production. The problem is that if the client add a static route toward the core switch bypassing both LTM and Firewall, the server stops to terminate the sessions with RST,ACK and all is ok. PS: we have NOT seen the ACK and RST packet in any F5 Capture, only on the destination server Capture. So either we can't seen the ACK bacause is locally generated by the LTM (correct?) but in this case we must at least see the RST sent by the server, or is not the LTM that send those packet. Thanks