How do I use a Mongodb aggregation variable such as the ones defined using `let` in $text operator search?

Question

So, I have indexed a field in "terms" collection, and I am performing a lookup in that collection. Everything is working perfectly except that I am not able to use the defined variable for searching purposes.

        {
            "$lookup": {
                "from": "terms",
                "as": "matched_terms",
                "let": {"localKeywordText": "$keywordText"},
                "pipeline": [
                    {"$match": {"$text": {"$search": "after birth bath soak"}}},
                    {"$project": {"text": 1, "match_criteria": 1, "account_id": 1, "_id": 0}},
                    {"$sort": {"score": {"$meta": "textScore"}}},
                    {"$match": {"account_id": object_id}},
                    {"$limit": 5}
                ]
            }
        },

The above query is running as expected, but I want to use the $$localKeywordText variable to perform the search operation. Putting the $text operator inside an $expr lead to the error:

Unrecognized expression '$text', full error: {'ok': 0.0, 'errmsg': "Unrecognized expression '$text'", 'code': 168, 'codeName': 'InvalidPipelineOperator'}