TCP Connection Failed After Adding New Members

What is the latency between F5 and the old/new pool members ? Normally, you can use WAN profile for the client side and LAN profile for the server side. If the latency is too high, you can try and use the WAN profile on both client and server side as a check.

If you disable the old pool members, are all connections successful ? I am wondering if your network set up may require SNAT by any chance.