The demo course on a sandbox comes with a welcome message with the date of the sandbox creation. On 's sandbox, this message is not displayed when you go to the Updates tool.
In Studio, this first message was created on 8/15, and a new message was created on 8/15, and only the new message appears. I also created two messages on 8/16, and both of those appear, so it is not simply that the code only supports one message per day.
I did not try to replicate this on a new sandbox.
Understand how this bug is happening
If it is only affecting the demo course and our sandboxes, let it drop