Error when bulk inserting documents with Cosmos DB (Response status code does not indicate success: Conflict (409); Substatus: 0; ...)

I am trying to add a collection of this object:

public class Organization
{
	[JsonProperty("id")]
	public string? Id { get; set; }
	[JsonProperty("name")]
	public string? Name { get; set; }
	[JsonProperty("address")]
	public string? Address { get; set; }
	[JsonProperty("createdOn")]
	public DateTime CreatedOn { get; set; }
	[JsonProperty("updatedOn")]
	public DateTime UpdatedOn { get; set; }
	[JsonProperty("owner")]
	public User? Owner { get; set; }
	[JsonProperty("creator")]
	public User? Creator { get; set; }
	[JsonProperty("lastUpdatedBy")]
	public User? LastUpdatedBy { get; set; }
	[JsonProperty("tenantId")]
	public string? TenantId { get; set; }
}

This is my code to bulk insert the documents:

public async Task<BulkOperationResponse<Organization>> BulkCreateOrganizationDB(List<Organization> reqs)
{
	BulkOperations<Organization> bulkOperations = new();

	foreach (var req in reqs)
	{
		PartitionKey partitionKey = new(req.TenantId);
		bulkOperations.Tasks.Add(_container.CreateItemAsync(req, partitionKey));
	}

	BulkOperationResponse<Organization> bulkResponse = await bulkOperations.ExecuteAsync();

	return bulkResponse;
}

But I get the following error that does not provide much information

Response status code does not indicate success: Conflict (409); Substatus: 0; ActivityId: 1cc2c968-2ccd-4a8d-8207-1a9b71b89d33; Reason: ();

I am using tenantId as the Partition Key in Cosmos DB..

I also tried the same approach for bulk update, and it worked correctly. How can I resolve this issue with bulk insert?