That may be explained by 8. Changes to the Default Formats in the Folder Formats Detailed FAQ.
It's one of the folder format behaviours which can seem strange/annoying in some situations, but if it didn't happen it would be annoying in other situations.
Also, the last point of the Folder Formats Quick Guide:
[quote]To test what you've done, close any open Opus windows, then open a new window and see if it is how you want it.
Always use a new window to check your changes.[/quote]