Because F5 is not a router, when you create a virtual server it accepts traffic which has the destination of that virtual server.
When you ping another interface other than where you come from it will not reply. I think it is normal.
"LTM is considered to be default deny. This means that when no traffic processing objects are configured, for example, a virtual server and a pool, the BIG-IP system will not process any network traffic. You need to configure at least one traffic processing object on the BIG-IP system to begin processing traffic."
A selfip is only reachable when the request is received on the VLAN on which it is configured.
So you cannot ping a selfip through the BIG-IP even if you have a forwarding VS.