This appears to be a bug. I can reproduce this in AB view (
and sometimes in Outline view), but not in other views such as Map or Chart. What I see is that the additional clauses of the Display Expression are appended to the note title. Thus for note $Name 'PDC' the $Name becomes
PDC : Agenda Items(2) : 20/08/2017
with the $DisplayName
PDC : Agenda Items(2) : 20/08/2017 : Agenda Items(2) : 20/08/2017
If we do the same again it becomes
PDC : Agenda Items(2) : 20/08/2017 : Agenda Items(2) : 20/08/2017: Agenda Items(2) : 20/08/2017
The $DisplayExpression code is not affected and $DisplayName is the evaluated string value of $DisplayExpression.
The error is not as simple as setting $Name to $DisplayName value when the selected item exits title-edit mode. If it did, we'd expect 'PDC' to get repeated, but it isn't.
The text pane title bar (right side, top of the pane) shows $Name - the actual note title. Editing that to remove the extra text is the easiest way to fix the result of this error pending a fix to the causal glitch.
Edit: on further testing this doesn't happen in Outline view: I'd misinterpreted events)