When I freeze rows/columns, the setting is not retained when I reopen the file. Is there something I am missing?

After I freeze rows / columns in a spreadsheet page, it holds for that session. However, after I have closed the spreadsheet, when I reopen it the setting is not retained. I have to set it again for that session. Is there something I have to do other than save the file to have this setting retained, or should I submit an enhancement request?

What is the file’s format?