Hi @li88li ,
In this 'Get message' action, there are several possibilities for the output to be empty:
1. Deleted message.
2. Unsupported content, such as 'Post message in a chat or channel', when retrieved by get message again, the display is empty.
I suggest you limit the message content to text and filter messages with empty content.
Please try the following workaround If it helps.
Edit the Filter array in advanced mode.

@and(and(not(equals(item()?['body/content'], '')),equals(item()?['body/contentType'], 'text')),equals(item()?['body/contentType'], 'text'))
Best Regards,
Zhi Chen