I have posted a video that shows the issue.  If I use the string builder for a textbox value and select a nested property, such as DayOfWeek, for a date, then it adds:

=Fields.DayOfWeek

which fails on Preview and is wrong.  Instead it should add:

=Fields.Birthday.DayOfWeek

Please fix this.  I know that I can go type ".Birthday" to fix the issue, but it really should add the nested properties properly.