The following line of code is affecting the Form's filter property.
Code:
DoCmd.OpenForm "frmRenewalActivityNotes", , , strWhere, , acDialog
If you execute that code, the form will open in Form View. If you Save the form, you will save the Filter property that the code assigned. I do not believe you need to switch to Design View prior to saving.
Your screenshot indicates the filter property was saved after your code was executed. I suggest deleting the text in the Field for the Filter property.
Having said that, subsequent execution of the Docmd code that includes the Filter argument will override a previously saved Filter assignment.
As for ActivityDate and ActivityNotes to be blank, you can make your String a little longer. Maybe something like ...
Code:
strWhere = "[PropertyID] = " & Me.PropertyID & " AND [ActivityDate] Is Null AND ([ActivityNotes] Is Null OR [ActivityNotes] = '')"