I would not be surprised if Apple is using some private API’s in the Files App.
Since Nitro works fine on some NAS brands, the difference between them is IMHO the Samba configuration of the NAS.
I have been looking in the smb configuration files of my Synology box and the only Vfs config line I see is : vfs objects=dirsort
I have minimal SMB version 2 and Max version 3.
I cannot find anything on specific issue with iPadOS and uGreen or empty SMB folders.
Do other applications have the same issues ?
A workaround might be ‘Secure Shellfish’, it can make sshfs connection available to other apps, as were they mapped SMB shares, using the ‘Browse for File’ UI.
I do not know if it works with the way Nitro requests the information, it uses the same API as cloud sync apps, as it is listed in that heading and not as an external server connection.