If you double click on the NET WORTH on the summary page it opens a report.
This report is base on the settings in the standard (not memorised) NET WORTH report in the Side Bar. Open the Side Bar NET WORTH report in the Side Bar click EDIT and check that all items that be included/excluded are correct. The Summary page NET WORTH includes Bank. Credit, Investment accounts, etc.
This explains why the problem exists, the net worth total should be defaulting to "as of today" date. There doesn't seem to be a way to set this default?
I am quite sure previous versions of Moneydance did not work like this.
If you set the Date in the NET WORTH (non-memorised) report to a date in the past that’s the date its users and remains at. If its set todays date then at midnight the date rolls forward thus remaining current.