Unexpected SSL Client Behavior

The BIG-IP should always use the server cipher list as the sole authority on the order of chosen ciphers (https://support.f5.com/csp/article/K12390). Given that, I suspect that the clients are not offering up the stronger ciphers as available. If the clients are offering up the same client hello, the BIG-IP will choose the same way every time.

You can see the list of ciphers offered by the client in a packet capture by looking at "Client Hello" packet in wireshark. It will be the first packing after the TCP handshake is complete.