Compound index on an array field is not being used correctly

In the following schema: 

{

    "arr" : [ 

        "a", 

        "b", 

        "c"

    ],

    "field" : 1,

    "field_2" : 1

}

with the following index: 

{

    "arr" : 1,

    "field" : 1,

    "field_2" : 1

}

If I use this query:

db.coll

    .find({

        $or: [

            {arr: 'a'},

            {arr: []}

        ]

    })

    .sort({field: 1})

    .explain()

The inputStages are separated to 3 stages - arr: [a, a], arr: [[], []], arr: [undefined, undefined].
This is a good scenario since these input stages are followed by a "SORT_MERGE" stage

BUT, if I use the following query:

db.coll

    .find({

        $or: [

            {arr: 'a'},

            {arr: []}

        ],

        field: 1

    })

    .sort({field_2: 1})

    .explain()

There are only 2 input stages - arr: [a, a], arr: [[undefined, undefined], [[] , []]] .
This results that an additional stage needs to happen in order to FETCH the empty array docs and then it cannot use the SORT_MERGE stage.

In large collection, this causes the following error:
"Sort operation used more than the maximum 33554432 bytes of RAM. Add an index, or specify a smaller limit
Even though there is an index for this query.

I would expect the second scenario to perform like the first one - separating the input stages to 3 stages and to use the SORT_MERGE function.