Packets to even numbered IP addresses vanish inside SRX 550

Scenario:

- upstream QFX pair, connected to SRX HA-pair, 4 ports crossed over for mesh redundancy (reth on SRX to ae on QFX)

- reth has LACP active

Problem:

- ping from QFX to host behind SRX only works for odd numbered IP address

Using capture files, I can see:

- ICMP ECHO-REQUEST arrive on reth from QFX (both odd and even numbered IPs)

- ICMP ECHO-REQUEST sent out physical (for odd numbered IPs)

- ICMP ECHO-REPLY arrive on the other side reth (for odd numbered IPs)

- ICMP ECHO-REPLY sent out physical to QFX (for odd numbered IPs)

Even numbered IP packets disappear.

LACP output:

    LACP state:       Role   Exp   Def  Dist  Col  Syn  Aggr  Timeout  Activity
      ge-0/0/6       Actor    No    No   Yes  Yes  Yes   Yes     Fast    Active
      ge-0/0/6     Partner    No    No   Yes  Yes  Yes   Yes     Fast   Passive
      ge-0/0/7       Actor    No    No   Yes  Yes  Yes   Yes     Fast    Active
      ge-0/0/7     Partner    No    No   Yes  Yes  Yes   Yes     Fast   Passive
      ge-9/0/6       Actor    No    No   Yes  Yes  Yes   Yes     Fast    Active
      ge-9/0/6     Partner    No    No   Yes  Yes  Yes   Yes     Fast   Passive
      ge-9/0/7       Actor    No    No   Yes  Yes  Yes   Yes     Fast    Active
      ge-9/0/7     Partner    No    No   Yes  Yes  Yes   Yes     Fast   Passive
    LACP protocol:        Receive State  Transmit State          Mux State
      ge-0/0/6                  Current   Fast periodic Collecting distributing
      ge-0/0/7                  Current   Fast periodic Collecting distributing
      ge-9/0/6                  Current   Fast periodic Collecting distributing
      ge-9/0/7                  Current   Fast periodic Collecting distributing

Interfaces are all up:

show interfaces terse | match reth6
ge-0/0/6.0              up    up   aenet    --> reth6.0
ge-0/0/7.0              up    up   aenet    --> reth6.0
ge-9/0/6.0              up    up   aenet    --> reth6.0
ge-9/0/7.0              up    up   aenet    --> reth6.0
reth6                   up    up
reth6.0                 up    up   inet     1.2.3.4/29

The reth looks like this:

show interfaces reth6
Physical interface: reth6  , Enabled, Physical link is Up
  Interface index: 134, SNMP ifIndex: 573
  Link-level type: Ethernet, MTU: 1514, Speed: 2Gbps, BPDU Error: None, MAC-REWRITE Error: None, Loopback: Disabled, Source filtering: Disabled, Flow control: Disabled, Minimum links needed: 1,
  Minimum bandwidth needed: 1bps
  Device flags   : Present Running
  Interface flags: SNMP-Traps Internal: 0x0
  Current address: 00:10:db:ED:IT:ED, Hardware address: 00:10:db:ED:IT:ED
  Last flapped   : 2018-12-10 17:08:10 EST (07:07:20 ago)
  Input rate     : 3936 bps (0 pps)
  Output rate    : 272 bps (0 pps)

  Logical interface reth6.0 (Index 95) (SNMP ifIndex 574)
    Flags: Up SNMP-Traps 0x0 Encapsulation: ENET2
    Statistics        Packets        pps         Bytes          bps
    Bundle:
        Input :        135576          0      16490480         3936
        Output:         58629          0       4452718          272
    Adaptive Statistics:
        Adaptive Adjusts:          0
        Adaptive Scans  :          0
        Adaptive Updates:          0
    Security: Zone: UNTRUSTED
    Allowed host-inbound traffic : ping ssh
    Protocol inet, MTU: 1500
      Flags: Sendbcast-pkt-to-re
      Addresses, Flags: Is-Preferred Is-Primary

What it feels like is the juniper is discarding the packets because it doesn't like which interface they arrived on.

reth6 is configured:

show configuration interfaces reth6
redundant-ether-options {
    redundancy-group 6;
    lacp {
        active;
    }
}
unit 0 {
    family inet {
        filter {
            input ICMP-CAPTURE;
            output ICMP-CAPTURE;
        }
        address 1.2.3.4/29;
    }
}

show configuration interfaces ge-0/0/6
gigether-options {
    redundant-parent reth6;
}
...

which I imagine to be pretty normal and the ge's are all the same.

I feel like I'm missing something obvious but I don't quite know what.