Standby Appliance receives HTTP Requests.

are you using connection mirroring?

e.g.

 configuration

[root@ve11a:Active:In Sync] config  tmsh list ltm virtual bar
ltm virtual bar {
    destination 172.28.24.10:80
    ip-protocol tcp
    mask 255.255.255.255
    mirror enabled
    pool foo
    profiles {
        http { }
        tcp { }
    }
    rules {
        qux
    }
    source 0.0.0.0/0
    source-address-translation {
        type automap
    }
    vs-index 12
}
[root@ve11a:Active:In Sync] config  tmsh list ltm rule qux
ltm rule qux {
    when HTTP_REQUEST {
  log local0. "hostname: [info hostname] \
    active: [HA::status active] standby: [HA::status standby] \
    client: [IP::client_addr]:[TCP::client_port]"
}
when HTTP_RESPONSE {
  log local0. "hostname: [info hostname] \
    active: [HA::status active] standby: [HA::status standby] \
    client: [IP::client_addr]:[TCP::client_port]"
}
}

 active

root@(ve11a)(cfg-sync In Sync)(Active)(/Common)(tmos) show ltm virtual bar raw

 (raw)
------------------------------------------------------------------
Ltm::Virtual Server: bar
------------------------------------------------------------------
Status
  Availability     : unknown
  State            : enabled
  Reason           : The children pool member(s) either don't have service checking enabled, or service check results are not available yet
  CMP              : enabled
  CMP Mode         : all-cpus
  Destination      : 172.28.24.10:80

Traffic                             ClientSide  Ephemeral  General
  Bits In                                 3312          0        -
  Bits Out                                4320          0        -
  Packets In                                 6          0        -
  Packets Out                                5          0        -
  Current Connections                        0          0        -
  Maximum Connections                        1          0        -
  Total Connections                          1          0        -
  Evicted Connections                        0          0        -
  Slow Connections Killed                    0          0        -
  Min Conn Duration/msec                     -          -      169
  Max Conn Duration/msec                     -          -      169
  Mean Conn Duration/msec                    -          -      169
  Total Requests                             -          -        1

SYN Cookies
  Status                         not-activated
  Hardware SYN Cookie Instances              0
  Software SYN Cookie Instances              0
  Current SYN Cache                          0
  SYN Cache Overflow                         0
  Total Software                             0
  Total Software Accepted                    0
  Total Software Rejected                    0
  Total Hardware                             0
  Total Hardware Accepted                    0

CPU Usage Ratio (%)
  Last 5 Seconds                             0
  Last 1 Minute                              0
  Last 5 Minutes                             0

[root@ve11a:Active:In Sync] config  tail -f /var/log/ltm
Dec 21 23:14:27 ve11a info tmm1[14890]: Rule /Common/qux : hostname: ve11a.acme.local  active: 1 standby: 0  client: 172.28.24.1:57671
Dec 21 23:14:27 ve11a info tmm1[14890]: Rule /Common/qux : hostname: ve11a.acme.local  active: 1 standby: 0  client: 172.28.24.1:57671

 standby

root@(ve11b)(cfg-sync In Sync)(Standby)(/Common)(tmos) show ltm virtual bar raw

 (raw)
------------------------------------------------------------------
Ltm::Virtual Server: bar
------------------------------------------------------------------
Status
  Availability     : unknown
  State            : enabled
  Reason           : The children pool member(s) either don't have service checking enabled, or service check results are not available yet
  CMP              : enabled
  CMP Mode         : all-cpus
  Destination      : 172.28.24.10:80

Traffic                             ClientSide  Ephemeral  General
  Bits In                                 3312          0        -
  Bits Out                                   0          0        -
  Packets In                                 6          0        -
  Packets Out                                0          0        -
  Current Connections                        0          0        -
  Maximum Connections                        1          0        -
  Total Connections                          1          0        -
  Evicted Connections                        0          0        -
  Slow Connections Killed                    0          0        -
  Min Conn Duration/msec                     -          -      268
  Max Conn Duration/msec                     -          -      268
  Mean Conn Duration/msec                    -          -      268
  Total Requests                             -          -        1

SYN Cookies
  Status                         not-activated
  Hardware SYN Cookie Instances              0
  Software SYN Cookie Instances              0
  Current SYN Cache                          0
  SYN Cache Overflow                         0
  Total Software                             0
  Total Software Accepted                    0
  Total Software Rejected                    0
  Total Hardware                             0
  Total Hardware Accepted                    0

CPU Usage Ratio (%)
  Last 5 Seconds                             0
  Last 1 Minute                              0
  Last 5 Minutes                             0

[root@ve11b:Standby:In Sync] config  tail -f /var/log/ltm
Dec 21 23:14:27 ve11b info tmm1[14917]: Rule /Common/qux : hostname: ve11b.acme.local  active: 0 standby: 1  client: 172.28.24.1:57671
Dec 21 23:14:27 ve11b info tmm1[14917]: Rule /Common/qux : hostname: ve11b.acme.local  active: 0 standby: 1  client: 172.28.24.1:57671