I didn’t see a relevant post on this, so I wanted to add it in case it is not a feature but rather a bug. Essentially, the Snap install on Ubuntu (18.04) sets the syncthing default directory at ~/snap/syncthing/ rather than ~/. Moreover, it puts the config files in ~/snap/syncthing/.config rather than ~/.config. The result, for me, was that it did not play nice with syncthing-gtk. Also, I believe it did not install the usual syncthing binaries at /usr/bin/syncthing. This led to errors when trying to use syncthing-gtk.
FWIW, a simple sudo apt install syncthing worked and everything went in the expected locations.