I setup a custom SaveFileDialog, in case there’s a difference from ShowSaveFileDialog as Julian used. And I tried having the .whatever folder at the beginning, middle and end fo the path. But all attempts worked correctly as expected.
I agree with Julian, you must be doing something else that you didn’t include in your description.
P.S. A period is not a problem on Macs. It’s just if the file or folder begins with a period then the file or folder is hidden. No different than setting the Hidden attribute on Windows.