Just now reproduced this behavior myself. Apparently this frontend validation-check, makes it skip the on-changeevent of other fields. But wait… after entering a text in ‘Message’ it works again, but removing the text no longer makes it fail. And I no longer can reproduce your problem. So it is an odd case that only happens if you follow a certain path of creating your page.
So to solve, do some wiggling. Add the Message, change the Event-settings, etc. It will start working again.
This is a bug though. If you can reproduce this, report it to Mendix at firstname.lastname@example.org or support.mendix.com.