The default PureFlow folder should be in iCloud -> PureFlow.
The first screen of the app is controlled by the iOS, not the PureFlow app (It is basically the Files app).
PureFlow does not have the ability to change the start page in this screen or manipulate files of folders on your device (except for the opened document of cause). There are no settings to change this. It should automatically open the location you used last.
If you can't find the folder you can try using search in PureFlow or in the Files app. Also check if you are logged in in iCloud with the correct account. I did not encounter such a problem before and I don't have any ideas what could have happened to the folders. As I said PureFlow does not have the ability to delete or move folders on the device.