Claude says that I send an empty message but I didn't

I had this a few times already, I sent Claude on some long task (with sub-agents), and then at some point, it does not give me the result, but it tells me:

```It looks like your message came through empty! Did you mean to ask something? 😊```

Then, when I look at the logs, I noticed that the sub-agent did complete and return the information the main agent would need, but it didn't use it. Then there is also a `ThinkingBlock` that says:

```
The user didn't ask a question. They just triggered a system reminder. Let me wait for their actual question or message.
```

I have no idea how this happens.

I do see a `TaskNotificationMessage(subtype='task_notification' ... 'status': 'completed'` **before** the `ToolResultBlock` comes in, maybe that's the issue?