Some flow rules are missing in CHO even intents are in INSTALLED state

in CHO test, there is a high percentage of ping failure after link up/down. Debugging reveals the following:
env: 600 host intent pairs, there are two pair of failure to ping.
In the details case, between h21 (on device 15/1) and h24(device 18/1).
1) Intents and flows are all in "INSTALLED" or "ADDED" states - correct;
2) path are fine:
onos> paths of:0000000000000015 of:0000000000000018
of:0000000000000015/3-of:0000000000000012/8==>of:0000000000000012/9-of:0000000000000016/4==>of:0000000000000016/6-of:0000000000000017/5==>of:0000000000000017/6-of:0000000000000018/2; cost=4.0

3) tshark tracing of ping packets reveal, of:16 not forwarding ping packets correctly;

4) check of:16 flows to reveal wrong flows have been installed - see the below 2nd flow, OUPORT should be 4, but it was 2.
onos> flows any of:0000000000000016 |grep -A 1 -B 2 "00:00:00:00:00:18" |grep -A 1 -B 2 "00:00:00:00:00:15"
treatment=DefaultTrafficTreatment{immediate=[OUTPUT

{port=5}

], deferred=[], transition=None, cleared=false, metadata=null}
id=2d000046641351, state=ADDED, bytes=75068, packets=766, duration=4000, priority=100, tableId=0 appId=org.onosproject.net.intent, payLoad=null
selector=[IN_PORT

{port=4}, ETH_DST{mac=00:00:00:00:00:18}, ETH_SRC{mac=00:00:00:00:00:15}]
treatment=DefaultTrafficTreatment{immediate=[OUTPUT{port=6}], deferred=[], transition=None, cleared=false, metadata=null}
–
treatment=DefaultTrafficTreatment{immediate=[OUTPUT{port=4}

], deferred=[], transition=None, cleared=false, metadata=null}
id=2d00004664138f, state=ADDED, bytes=75264, packets=768, duration=4000, priority=100, tableId=0 appId=org.onosproject.net.intent, payLoad=null
selector=[IN_PORT

{port=6}

, ETH_DST

{mac=00:00:00:00:00:15}

, ETH_SRC

{mac=00:00:00:00:00:18}

]
treatment=DefaultTrafficTreatment{immediate=[OUTPUT

{port=2}

], deferred=[], transition=None, cleared=false, metadata=null}

5) tshark also confirm that the flow was indeed direct to port 2 of OF:16 device.