ServiceBus MessageLock Exception if awaiting all CompleteAsync tasks at once

Using Azure.Messaging.ServiceBus v7.3.0. When calling ServiceBusReceiver.CompleteMessageAsync(), storing the resulting task into a collection and then awaiting all with await Task.WhenAll(completionTasks); eg:

foreach (var message in messagesToComplete)
{
  completionTasks.Add(ServiceBusReceiver.CompleteMessageAsync(message));
}
await Task.WhenAll(completionTasks);

This will result in a MessageLock exception when running on Azure (and sometimes locally):

2021-09-16T18:48:00Z   [Information]   Attempting to complete 576 messages.
2021-09-16T18:48:04Z   [Error]   Exception while completing messages: Azure.Messaging.ServiceBus.ServiceBusException: The lock supplied is invalid. Either the lock expired, or the message has already been removed from the queue, or was received by a different receiver instance. (MessageLockLost)
2021-09-16T18:48:04Z   [Verbose]   Oldest message is locked until: 09/16/2021 18:48:30 +00:00
2021-09-16T18:48:04Z   [Verbose]   Youngest message is locked until: 09/16/2021 18:48:30 +00:00

Note the timestamp of when that happened and the locked until messages below. They still had 26 seconds before timing out.

Switching to looping through the collection of message to complete and awaiting each one, eg:

foreach (var message in messagesToComplete)
{
  await ServiceBusReceiver.CompleteMessageAsync(message);
}

This solved the issue and the locking behaves as expected.

[JoshLove-msft]

The dev investigating this is currently out of the office, but we should have an update next week.

[JoshLove-msft]

@DorothySun216 is investigating this - apparently 1 message gets throttled and all subsequent messages get removed from the unsettled map so they can no longer be settled.

Need to double check with the Track 2 SDK because when testing this I didn’t see a throttling exception - only lock lost.