Elasticsearch returns document missing error with doc_as_upsert

Question

I'm running Elasticsearch 6.2.4 and am occasionally getting an unexpected document missing exception when I try to index a new document using the bulk api.

Here's the job I'm sending:

{:update
 {:_id "3b4ff48814eff41bc4014c19",
  :_index "customerAlias",
  :_type "doc",
  :routing "customerAlias"}}
 {:doc
  {:id "3b4ff48814eff41bc4014c19"
   :some "data"},
  :doc_as_upsert true}

And the response I'm getting:

{:type "document_missing_exception",
 :reason "[doc][3b4ff48814eff41bc4014c19]: document missing",
 :index_uuid "KzcNGk1oTOijW00oDJDacQ",
 :shard "0",
 :index "prefix-customerAlias"}

I'd estimate that tens of thousands of documents per day go through this code path successfully, but a few times a day I get this error.

Under what circumstances will Elasticsearch complain that the document it's being asked to index, with doc_as_upsert, doesn't exist?