CosmosDB Upsert gives status code 449 even when there is no conflict - azure-cosmosdb

Question

A .NET 6 application Upserts documents to CosmosDB.
I get many debug messages (not all writes)
DocDBTrace Error: 0 : DocumentClientException with status code 449, message: Message: {"Errors":["Conflicting request to resource has been attempted. Retry to avoid conflicts."]}
the ids of the documents are Guid instances and there are no unique keys in the container (so no conflicts).
What could it be?
EDIT
partitionKey = id
//F#
let write database container items =
let container = client.GetContainer(database, container)
items
|> Seq.map (fun item -> container.UpsertItemAsync(item))
|> Seq.toArray
|> Task.WhenAll
Data sample
{
"RESPONSE": [
{
"IntradayTickResponse": {
"CorrelationID": "ESM2 Index:intraday",
"TimeReceived": "2022-05-05T01:19:38.338794Z",
"tickData": {
"eidData": null,
"tickData": [
{
"time": {
"DayOfMonth": 5,
"Hour": 0,
"MicroSecond": 0,
"MilliSecond": 0,
"Minute": 37,
"Month": 5,
"NanoSecond": 0,
"Parts": 2046,
"PicoSecond": 0,
"Second": 7,
"TimezoneOffsetMinutes": 0,
"Year": 2022
},
"type": "TRADE",
"value": 4292.5,
"size": 2,
"conditionCodes": "TSUM"
},
{
"time": {
"DayOfMonth": 5,
"Hour": 0,
"MicroSecond": 0,
"MilliSecond": 0,
"Minute": 37,
"Month": 5,
"NanoSecond": 0,
"Parts": 2046,
"PicoSecond": 0,
"Second": 19,
"TimezoneOffsetMinutes": 0,
"Year": 2022
},
"type": "TRADE",
"value": 4292.75,
"size": 2,
"conditionCodes": "TSUM"
},
{
"time": {
"DayOfMonth": 5,
"Hour": 0,
"MicroSecond": 0,
"MilliSecond": 0,
"Minute": 37,
"Month": 5,
"NanoSecond": 0,
"Parts": 2046,
"PicoSecond": 0,
"Second": 19,
"TimezoneOffsetMinutes": 0,
"Year": 2022
},
"type": "TRADE",
"value": 4292.75,
"size": 1,
"conditionCodes": "TSUM"
},
{
"time": {
"DayOfMonth": 5,
"Hour": 0,
"MicroSecond": 0,
"MilliSecond": 0,
"Minute": 37,
"Month": 5,
"NanoSecond": 0,
"Parts": 2046,
"PicoSecond": 0,
"Second": 19,
"TimezoneOffsetMinutes": 0,
"Year": 2022
},
"type": "TRADE",
"value": 4292.75,
"size": 1,
"conditionCodes": "TSUM"
},
{
"time": {
"DayOfMonth": 5,
"Hour": 0,
"MicroSecond": 0,
"MilliSecond": 0,
"Minute": 37,
"Month": 5,
"NanoSecond": 0,
"Parts": 2046,
"PicoSecond": 0,
"Second": 19,
"TimezoneOffsetMinutes": 0,
"Year": 2022
},
"type": "TRADE",
"value": 4292.75,
"size": 1,
"conditionCodes": "TSUM"
},
...
"id": "87fdc18a-61f2-4c43-a7a8-2904b14e6596",
"_rid": "maszAOGSZkYwAgAAAAAAAA==",
"_self": "dbs/maszAA==/colls/maszAOGSZkY=/docs/maszAOGSZkYwAgAAAAAAAA==/",
"_etag": "\"00000000-0000-0000-601e-3057eca901d8\"",
"_attachments": "attachments/",
"_ts": 1651713578
}