I've analyzed the AAPS source code in detail. Here's what I found about the data shape and patterns relevant to your MongoDB concerns:

AAPS Data Sending Pattern to Nightscout v3
Key Finding: AAPS Sends Documents One at a Time, Not in Batches
Looking at DataSyncSelectorV3.kt, AAPS processes records sequentially in a while loop, sending individual POST/PATCH requests for each document:

// Pattern from processChangedBoluses(), processChangedCarbs(), etc.
while (cont) {
    persistenceLayer.getNextSyncElementBolus(startId).blockingGet()?.let { bolus ->
        cont = activePlugin.activeNsClient?.nsAdd("treatments", ...) == true
        // waits for response before next iteration
    }
}
The API calls in NightscoutRemoteService.kt confirm single-document operations:

POST /v3/entries (single RemoteEntry)
POST /v3/treatments (single RemoteTreatment)
POST /v3/devicestatus (single RemoteDeviceStatus)
PATCH /v3/treatments/{identifier} (single document updates)
Data Structures (from examples.json and remote models)
1. Entries (CGM readings):

{
  "type": "sgv",
  "sgv": 98,
  "date": 1549414398005,
  "dateString": "2019-02-06T01:53:18.005+0100",
  "device": "AndroidAPS-DexcomG6",
  "direction": "Flat",
  "identifier": "5c5a3007e0196f4d3d9aeafc",
  "srvModified": 1549414398005,
  "srvCreated": 1549414398005,
  "utcOffset": 120,
  "app": "AAPS"
}
2. Treatments (boluses, carbs, temp basals, etc.):

{
  "eventType": "Meal Bolus",
  "insulin": 8.1,
  "created_at": "2021-07-13T11:25:12.664Z",
  "date": 1626175512664,
  "type": "NORMAL",
  "isValid": true,
  "isSMB": false,
  "pumpId": 4102,
  "pumpType": "ACCU_CHEK_INSIGHT_BLUETOOTH",
  "pumpSerial": "33013206",
  "identifier": "60ed782dc574da0004a38595",
  "app": "AAPS"
}
3. Deduplication Keys (critical for conflict resolution):
AAPS uses a composite key for deduplication:

identifier - Server-assigned document ID
pumpId + pumpType + pumpSerial - Unique pump event identification
srvModified - Timestamp for conflict detection
Suggested Test Fixtures for Nightscout Core Team
Based on the AAPS patterns, here are test scenarios for MongoDB updates handling multiple documents:

Fixture 1: High-frequency SGV uploads (every 5 min):

[
  {"type": "sgv", "sgv": 120, "date": 1705600000000, "device": "AAPS", "direction": "Flat"},
  {"type": "sgv", "sgv": 125, "date": 1705600300000, "device": "AAPS", "direction": "FortyFiveUp"},
  {"type": "sgv", "sgv": 130, "date": 1705600600000, "device": "AAPS", "direction": "SingleUp"}
]
Fixture 2: Burst of SMB boluses (common in closed-loop):

[
  {"eventType": "Correction Bolus", "insulin": 0.25, "isSMB": true, "pumpId": 4148, "pumpType": "ACCU_CHEK_INSIGHT_BLUETOOTH", "pumpSerial": "33013206"},
  {"eventType": "Correction Bolus", "insulin": 0.30, "isSMB": true, "pumpId": 4149, "pumpType": "ACCU_CHEK_INSIGHT_BLUETOOTH", "pumpSerial": "33013206"},
  {"eventType": "Correction Bolus", "insulin": 0.15, "isSMB": true, "pumpId": 4150, "pumpType": "ACCU_CHEK_INSIGHT_BLUETOOTH", "pumpSerial": "33013206"}
]
Fixture 3: Mixed treatment types (meal scenario):

[
  {"eventType": "Meal Bolus", "carbs": 45, "date": 1626200383325, "isValid": true},
  {"eventType": "Meal Bolus", "insulin": 8.1, "pumpId": 4102, "type": "NORMAL"},
  {"eventType": "Bolus Wizard", "bolusCalculatorResult": "{...}", "glucose": 134}
]
Fixture 4: Deduplication test (same pumpId sent twice):

[
  {"eventType": "Correction Bolus", "pumpId": 4148, "pumpType": "DANA_R", "pumpSerial": "12345"},
  {"eventType": "Correction Bolus", "pumpId": 4148, "pumpType": "DANA_R", "pumpSerial": "12345"}
]
MongoDB Concern Insight
Since AAPS sends documents one at a time (not bulk inserts), the MongoDB concern about "handling multiple documents" likely arises from:

Rapid sequential requests during sync catch-up (hundreds of documents in quick succession)
Deduplication logic when isDeduplication: true is returned