The test unionWith_allows_stages.js fails sporadically for sharded collections because the $indexStats stage of the following pipeline only runs in one shard when it's expected to run on all of the shards of the cluster (2 in total).

let docArray = collA
    .aggregate([
        {$match: {val: {$exists: false}}},
        {$unionWith: {coll: textColl.getName(), pipeline: [{$indexStats: {}}]}},
        {$sort: {name: 1}},
    ])
    .toArray();