By declaring in runtime.json a linux.resources like the following:

"resources": {
    [omitted]
    "blockIO": {
        "blkioWeight": 0,
        "blkioWeightDevice": "8:0 500\n8:16 500",
        "blkioThrottleReadBpsDevice": "",
        "blkioThrottleWriteBpsDevice": "",
        "blkioThrottleReadIopsDevice": "",
        "blkioThrottleWriteIopsDevice": ""
    },
    [omitted]
},

blkio.weight_device (and blkioThrottle*) isn't populated with both the devices but only the first one is correctly added.
Current code is directly writing 8:0 500\n8:16 500 to the cgroup file which isn't clearly working. To fix this, multiple ioutil.WriteFile() should be done for each device declared.
The same applies to the others fields defined above as a string.

proposed spec changes opencontainers/runtime-spec#193
this PR moby/moby#13959 and two other are wrongly assuming the above format will work

Once/If the proposed changes to the spec lands I'll gladly take care of this here if it's ok