repro.js

/* 
 * start mongocryptd, then do:
 * mongo --port 27020 repro.js
 */
var schema = {
  properties: {
    ssn: {
      encrypt: {
        keyId: [
            BinData(4, "AAAAAAAAAAAAAAAAAAAAAA==")
        ],
        bsonType: "string",
        algorithm: "AEAD_AES_256_CBC_HMAC_SHA_512-Deterministic",
        initializationVector: BinData(0, "aWlpaWlpaWlpaWlpaWlpaQ==")
      }
    }
  },
  bsonType: "object"
};

var update_cmd =
{
  update: "default",
  updates: [
     { q: {},
       u: { $set: {"y.$[i].b": 2} },
       arrayFilters: [ {"i.b": 4} ]
     }
  ],
  ordered: true,
  jsonSchema: schema
}

printjson(db.runCommand(update_cmd))

/*
Even though "y" is not an encrypted field, we get back:
{
	"ok" : 0,
	"errmsg" : "No array filter found for identifier 'i' in path 'y.$[i].b'",
	"code" : 2,
	"codeName" : "BadValue"
}
*/