Changes to pool priority groups

We have recently upgraded a pair of our Viprions from 10.2.4 to 11.4. Since that time one of our services is behaving oddly.

We have 2 priority groups in a pool. One service goes up and down througout the day, the higher priority one. However it seems that the LB is occasionally sending traffic to the secondary priority group eventhough the server is the higher priority group is still running. Here is the configs...

ltm pool /Common/PROD-pool-GL-Snap-54700 {
    load-balancing-mode fastest-node
    members {
        /Common/10.129.136.21:54700 {
            address 10.129.136.21
            priority-group 2
        }
        /Common/10.129.136.22:54700 {
            address 10.129.136.22
            priority-group 2
        }
        /Common/10.130.40.33:54700 {
            address 10.130.40.33
            priority-group 2
        }
        /Common/10.130.40.34:54700 {
            address 10.130.40.34
            priority-group 2
        }
        /Common/10.130.40.41:54700 {
            address 10.130.40.41
            priority-group 2
        }
        /Common/10.130.40.42:54700 {
            address 10.130.40.42
            priority-group 2
        }
        /Common/10.130.40.43:54700 {
            address 10.130.40.43
            priority-group 2
        }
        /Common/10.130.40.44:54700 {
            address 10.130.40.44
            priority-group 2
        }
        /Common/174.234.194.154:80 {
            address 174.234.194.154
            priority-group 1
        }
    }
    min-active-members 1
    monitor /Common/tcp_half_open
}

This only started happening after the upgrade. I don't see the service being marked down in the logs.