Unable to search for flattened (nested) field existence

I'm storing an arbitrary nested object as a flattened field "_meta" which contains various information related to a product.
Here is the mapping for that field:

"mappings": {
        "dynamic": "strict",
        "properties": {
            "_meta": {
                "type": "flattened"

So when trying to search for:

    "query": {
        "exists": {
            "field": "_meta.user"

I'm expecting to retrieve all documents that have that field populated. I get zero hits, although if I search for a particular document, I can see that at least one document has that field populated:

"user": {
  "origin_title": "some title",
  "origin_title_en": "some other title",
  "address": "some address",
  "performed_orders_count": 0,
  "phone": "some phone",
  "name": "some name",
  "tariff": null,
  "proposal_image_background_color": null

So how exactly does searching through a flattened data field work?
Why I'm not getting any results?

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