Load balance with Persistence on two virtual servers.(front end and back end)

Just spitballing here, but destination address affinity is good for things like load balancing firewalls or caching proxies, where the destination address is something forward of the load balancer. You have 5 API servers in a pool, so given that a persistence record is created on the first load balancing decision, and depending on timing you could potentially hit 100% on a single server. Have you tried source address persistence? At the very least that should spread the load across 8 persistence records.