Syncthing did not create the .stversions folder for any client. I changed the File versioning strategy, from trash can, simple and staggered and it never creates the .stversions.
The destination folder on server is owned by the syncthing user and group, so permissions are ok.
Server: Freenas - Syncthing Plugin version v0.14.45, FreeBSD (64 bit)
Client: Mac OX 10.11.6 El Capitan - Syncthing installed from Brew - Syncthing version v0.14.44, Mac OS X (64 bit)
Example
Client side:
Lucho$ touch test.txt
Lucho$ vi test.txt
Lucho$ echo "information" >> ./test.txt
Server side (after synchronization):
root@freenas:/mnt/Storage/fileserver/Lucianom/NICO # ls -la
total 34136
drwxr-xr-x 2 syncthing syncthing 6 Mar 23 19:37 .
drwxrwxrwx 36 syncthing syncthing 40 Mar 23 18:57 …
-rw-r–r-- 1 syncthing syncthing 6148 Mar 23 19:18 .DS_Store
-rw-r–r-- 1 syncthing syncthing 5 Mar 23 19:20 chau.txt
-rw-r–r-- 1 syncthing syncthing 34955475 Dec 13 09:48 TeamViewer.dmg
-rw-r--r-- 1 syncthing syncthing 17 Mar 23 19:37 test.txt
root@freenas:/mnt/Storage/fileserver/Lucianom/NICO # cat test.txt TEST information
root@freenas:/mnt/Storage/fileserver/Lucianom/NICO # ls -la ../ total 1238 drwxrwxrwx 36 syncthing syncthing 40 Mar 23 18:57 . drwxrwxrwx 9 syncthing syncthing 10 Mar 19 21:44 .. d--x--x--x 2 syncthing syncthing 2 Mar 19 18:42 .DocumentRevisions-V100 -rw-r--r-- 1 syncthing syncthing 10244 Mar 16 11:29 .DS_Store drwx------ 2 syncthing syncthing 2 Mar 19 18:42 .fseventsd drwx------ 2 syncthing syncthing 2 Mar 19 18:42 .Spotlight-V100 drwxr-xr-x 2 syncthing syncthing 2 Mar 19 18:41 .stfolder drwxrwxrwx 5 syncthing syncthing 5 Mar 19 18:42 .TemporaryItems d-wx-wx-wx 2 syncthing syncthing 2 Mar 19 18:42 .Trashes -rw-r--r-- 1 syncthing syncthing 154809 Oct 14 2016 .VolumeIcon.icns