Mongodb query does not use prefix on compound index with text field

I've created the following index on my collection:

db.myCollection.createIndex({
  user_id: 1,
  name: 'text'
})

If I try to see the execution plan of a query containing both fields, like this:

db.getCollection('campaigns').find({ 
    user_id: ObjectId('xxx')
   ,$text: { $search: 'bla' } 
}).explain('executionStats')

I get the following results:

...
"winningPlan" : {
    "stage" : "TEXT",
    "indexPrefix" : {
        "user_id" : ObjectId("xxx")
    },
    "indexName" : "user_id_1_name_text",
    "parsedTextQuery" : {
        "terms" : [ 
            "e"
        ],
        "negatedTerms" : [],
        "phrases" : [],
        "negatedPhrases" : []
    },
    "inputStage" : {
        "stage" : "TEXT_MATCH",
        "inputStage" : {
            "stage" : "TEXT_OR",
            "inputStage" : {
                "stage" : "IXSCAN",
                "keyPattern" : {
                    "user_id" : 1.0,
                    "_fts" : "text",
                    "_ftsx" : 1
                },
                "indexName" : "user_id_1_name_text",
                "isMultiKey" : true,
                "isUnique" : false,
                "isSparse" : false,
                "isPartial" : false,
                "indexVersion" : 1,
                "direction" : "backward",
                "indexBounds" : {}
            }
        }
    }
}
...

As stated in the documentation, MongoDB can use index prefixes to perform indexed queries.

Since user_id is a prefix for the index above, I'd expect that a query only by user_id would use the index, but if I try the following:

db.myCollection.find({ 
    user_id: ObjectId('xxx')
}).explain('executionStats')

I get:

...
"winningPlan" : {
    "stage" : "COLLSCAN",
    "filter" : {
        "user_id" : {
            "$eq" : ObjectId("xxx")
        }
    },
    "direction" : "forward"
},
...

So, it is not using the index at all and performing a full collection scan.