IP forwarding issue

It could be many things like translate address and port not dissabled, asymetric routing, traffic not comming on the right vlan etc.. Just as a test did you create a virtual server as described in https://support.f5.com/csp/article/K7595 under "Emulating stateless IP routing with BIG-IP LTM forwarding virtual servers" to see if there is no asymetric routing or something else causing the issue?

Also you may enable tcp rst and do a packet capture on the F5 with nnnp just to see if traffic is reaching the F5 device and if the F5 is dropping it for some reason and also check the /var/log/ltm if there are some errors :

https://support.f5.com/csp/article/K13223

https://support.f5.com/csp/article/K13637

https://support.f5.com/csp/article/K9812