$unionWIth $search explain errors out when using $search and $$SEARCH_META in the subpipeline since SEARCH_META is used in the original run of the pipeline which seems to add it in the let variables. Then when $unionWith reruns the pipeline for explain, it errors due to SEARCH_META not being allowed to be in the letVariables.

pipeline: [{
    $unionWith: {
        coll: searchColl.getName(),
        pipeline: [
            {$search: mongotQuery},
            {
                $project: {
                    "_id": 0,
                    "ref_id": "$_id",
                    "title": "$title",
                    "searchMeta": "$$SEARCH_META",
                }
            }
        ]
    }
}]

A possible solution is to clear SEARCH_META from the let variables.