Persistence between front end and back end servers

It's surprising that persistence uses the SNAT IP since persistence should be applied before SNAT in the BIG-IP. But that it's not doing persistence based on the client's IP is not surprising however, since it's the frontend servers that opens the second connection to the backend Virtual Server, and thus the Source IP should be either WebA or WebB, so doing source address persistence when you have only two possible sources will never work very well.

As for a solution, yes there is - use another persistence method. I would suspect that the application has some form of session ID included in the traffic, either in the form of a cookie or maybe as a parameter in the URI. There is a persistence method called universal persistence and the idea there is that you write an iRule that extracts the session ID from the traffic and then the universal persistence will persist on that value. So do you know how that session ID is sent (cookie or parameter or both)?