Unfortunately I cannot provide a sample.

But yes, for some reason, Saturday and Sunday are being included as start and finish dates despite using the standard Work Calendar.

I noticed the change after I included a lag on a Finish date to catch the schedule up with current dates. Other times I have simply entered a new date in either the Start or Finish columns.

I also notices that when I deleted a Predecessor for one of the tasks, the Start and Finish dates for the task reverted to a date in 2016. When I checked, this was related to the Project Start date found in "Project Information". I had not updated the start date when I used an older file to create a new one.

Would any of those things affect the use of non working days?