Ok, had a look, as I thought, there’s nothing we can do I’m afraid. What happens is that if you start typing the shortcut Agenda notices that you are changing the title and schedules an automatic save to happen in a few seconds. As no activity is detected (you are working inside a TextExpander window that we don’t have access to), the save happens and the field is reloaded. Not sure what TE does but it does somehow empty the field already, which means it revert to Untitled as we don’t allow for notes without titles.
One thing you could consider is to use the Template feature in Agenda, which allows you to prepare the content of a note for your Work Journal entry (it’s pretty much exactly the use case it was made for). You can already prepare the Date part of the title using text actions like \date as described here: Creating and Using Templates