This is my fist try to sync Synology NAS with Android.
In Synology NAS I have added a folder and shared it with my Pixel mobil and it looks OK.
In Pixel I have added a device with QR from Syncthing NAS and also defined the folders (local one which I want sync and destination folder at NAS)
When sync start it looks strange and as result I get a lot of strange subdir in the device-destination folder:
The primary use of the “untrusted” feature (which you enable by setting a password when you share a particular folder with a particular device) is, in my opinion, this: You want to share with a device which you trust enough to not throw away your files, but you don’t trust it enough to have clear text files there. Typically a device in a colo or at a friend.
Your phone is likely a device you trust, so when on the NAS you shared a folder with the phone, you should NOT have entered a password.