UpsertDocumentAsync api sometimes return null etag

[htzhang2]

Describe the bug
Occasionally(~1/50 times), UpsertDocumentAsync returns null etag without exception

To Reproduce
for (int i = 0; i <100; i++)
{
// Update document

try
{
Document doc = await this.DocumentDbClient.UpsertDocumentAsync(Uri, updatedDoc).ConfigureAwait(false);
Assert.IsNotNull(doc.ETag);
}
catch(Exception ex)
{
// Log exception
}
}

Expected behavior
ETag not null

Actual behavior
~1/50 ETag is null

Environment summary

SDK Version: 2.18.0
OS Version (e.g. Windows, Linux, MacOSX): Windows

Additional context
Azure Service Fabric
Azure CosmosDb