HAR requests missing "initiator" field

Support
1

For example: http://httparchive.webpagetest.org/result/181215_N4_199B9/

In HTTP Archive I’m seeing ~28% of all requests missing the “initiator” field. I would expect this field to exist almost every time. In the example above, the “initiator” is only set on the first request. This is the worst case scenario – I haven’t seen any tests with 0 initiators.

Here are more examples if needed:

[
  {
    "page": "https://bananas-store.com/",
    "wptid": "181215_EB_W0MV",
    "pct_null_initiator": "0.9986168741355463"
  },
  {
    "page": "http://ksnctf.sweetduet.info/",
    "wptid": "181215_4D_8N6N",
    "pct_null_initiator": "0.9978773084270856"
  },
  {
    "page": "http://noblefraud.pl/",
    "wptid": "181215_1VQ_3MNP1",
    "pct_null_initiator": "0.9978005865102639"
  },
  {
    "page": "http://freetoon01.com/",
    "wptid": "181215_GB_ZZ3H",
    "pct_null_initiator": "0.9972789115646259"
  },
  {
    "page": "https://ixiner.org/",
    "wptid": "181215_Z1_1WMV1",
    "pct_null_initiator": "0.9971563981042654"
  },
  {
    "page": "https://rise61.ru/",
    "wptid": "181215_DG_TDGK",
    "pct_null_initiator": "0.9971264367816092"
  },
  {
    "page": "https://www.alberghieroassisi.eu/",
    "wptid": "181215_KZ_1701W",
    "pct_null_initiator": "0.9970501474926253"
  },
  {
    "page": "http://www.next.cc/",
    "wptid": "181215_CR_RX3S",
    "pct_null_initiator": "0.9969788519637462"
  },
  {
    "page": "http://www.usedtrucks.mercedes-benz.co.uk/",
    "wptid": "181215_N4_199B9",
    "pct_null_initiator": "0.9967637540453075"
  },
  {
    "page": "http://www.sorozatos.webtelek.hu/",
    "wptid": "181215_GT_10W9S",
    "pct_null_initiator": "0.9966887417218543"
  }
]

Is this a bug in WPT/Chrome or are there some cases when the initiator is not expected?